ahci: do not request irq for dummy port

System may crash in ahci_hw_interrupt() or ahci_thread_fn() when
accessing the interrupt status in a port's private_data if the port is
actually a DUMMY port.

00:1f.2 SATA controller: Intel Corporation 82801JI (ICH10 Family) SATA AHCI Controller

<snip console output for linux-3.15-rc1>
[    9.352080] ahci 0000:00:1f.2: AHCI 0001.0200 32 slots 6 ports 3 Gbps 0x1 impl SATA mode
[    9.352084] ahci 0000:00:1f.2: flags: 64bit ncq sntf pm led clo pio slum part ccc
[    9.368155] Console: switching to colour frame buffer device 128x48
[    9.439759] mgag200 0000:11:00.0: fb0: mgadrmfb frame buffer device
[    9.446765] mgag200 0000:11:00.0: registered panic notifier
[    9.470166] scsi1 : ahci
[    9.479166] scsi2 : ahci
[    9.488172] scsi3 : ahci
[    9.497174] scsi4 : ahci
[    9.506175] scsi5 : ahci
[    9.515174] scsi6 : ahci
[    9.518181] ata1: SATA max UDMA/133 abar m2048@0x95c00000 port 0x95c00100 irq 91
[    9.526448] ata2: DUMMY
[    9.529182] ata3: DUMMY
[    9.531916] ata4: DUMMY
[    9.534650] ata5: DUMMY
[    9.537382] ata6: DUMMY
[    9.576196] [drm] Initialized mgag200 1.0.0 20110418 for 0000:11:00.0 on minor 0
[    9.845257] ata1: SATA link up 1.5 Gbps (SStatus 113 SControl 300)
[    9.865161] ata1.00: ATAPI: Optiarc DVD RW AD-7580S, FX04, max UDMA/100
[    9.891407] ata1.00: configured for UDMA/100
[    9.900525] scsi 1:0:0:0: CD-ROM            Optiarc  DVD RW AD-7580S  FX04 PQ: 0 ANSI: 5
[   10.247399] iTCO_vendor_support: vendor-support=0
[   10.261572] iTCO_wdt: Intel TCO WatchDog Timer Driver v1.11
[   10.269764] iTCO_wdt: unable to reset NO_REBOOT flag, device disabled by hardware/BIOS
[   10.301932] sd 0:2:0:0: [sda] 570310656 512-byte logical blocks: (291 GB/271 GiB)
[   10.317085] sd 0:2:0:0: [sda] Write Protect is off
[   10.328326] sd 0:2:0:0: [sda] Write cache: disabled, read cache: disabled, supports DPO and FUA
[   10.375452] BUG: unable to handle kernel NULL pointer dereference at 000000000000003c
[   10.384217] IP: [<ffffffffa0133df0>] ahci_hw_interrupt+0x100/0x130 [libahci]
[   10.392101] PGD 0
[   10.394353] Oops: 0000 [#1] SMP
[   10.397978] Modules linked in: sr_mod(+) cdrom sd_mod iTCO_wdt crc_t10dif iTCO_vendor_support crct10dif_common ahci libahci libata lpc_ich mfd_core mgag200 syscopyarea sysfillrect sysimgblt i2c_algo_bit drm_kms_helper ttm drm i2c_core megaraid_sas dm_mirror dm_region_hash
dm_log dm_mod
[   10.426499] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 3.15.0-rc1 #1
[   10.433495] Hardware name: QCI QSSC-S4R/QSSC-S4R, BIOS QSSC-S4R.QCI.01.00.S013.032920111005 03/29/2011
[   10.443886] task: ffffffff81906460 ti: ffffffff818f0000 task.ti: ffffffff818f0000
[   10.452239] RIP: 0010:[<ffffffffa0133df0>]  [<ffffffffa0133df0>] ahci_hw_interrupt+0x100/0x130 [libahci]
[   10.462838] RSP: 0018:ffff880033c03d98  EFLAGS: 00010046
[   10.468767] RAX: 0000000000a400a4 RBX: ffff880029a6bc18 RCX: 00000000fffffffa
[   10.476731] RDX: 00000000000000a4 RSI: ffff880029bb0000 RDI: ffff880029a6bc18
[   10.484696] RBP: ffff880033c03dc8 R08: 0000000000000000 R09: ffff88002f800490
[   10.492661] R10: 0000000000000000 R11: 0000000000000005 R12: 0000000000000000
[   10.500625] R13: ffff880029a6bd98 R14: 0000000000000000 R15: ffffc90000194000
[   10.508590] FS:  0000000000000000(0000) GS:ffff880033c00000(0000) knlGS:0000000000000000
[   10.517623] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[   10.524035] CR2: 000000000000003c CR3: 00000000328ff000 CR4: 00000000000007b0
[   10.531999] Stack:
[   10.534241]  0000000000000017 ffff880031ba7d00 000000000000005c ffff880031ba7d00
[   10.542535]  0000000000000000 000000000000005c ffff880033c03e10 ffffffff810c2a1e
[   10.550827]  ffff880031ae2900 000000008108fb4f ffff880031ae2900 ffff880031ae2984
[   10.559121] Call Trace:
[   10.561849]  <IRQ>
[   10.563994]  [<ffffffff810c2a1e>] handle_irq_event_percpu+0x3e/0x1a0
[   10.571309]  [<ffffffff810c2bbd>] handle_irq_event+0x3d/0x60
[   10.577631]  [<ffffffff810c4fdd>] try_one_irq.isra.6+0x8d/0xf0
[   10.584142]  [<ffffffff810c5313>] note_interrupt+0x173/0x1f0
[   10.590460]  [<ffffffff810c2a8e>] handle_irq_event_percpu+0xae/0x1a0
[   10.597554]  [<ffffffff810c2bbd>] handle_irq_event+0x3d/0x60
[   10.603872]  [<ffffffff810c5727>] handle_edge_irq+0x77/0x130
[   10.610199]  [<ffffffff81014b8f>] handle_irq+0xbf/0x150
[   10.616040]  [<ffffffff8109ff4e>] ? vtime_account_idle+0xe/0x50
[   10.622654]  [<ffffffff815fca1a>] ? atomic_notifier_call_chain+0x1a/0x20
[   10.630140]  [<ffffffff816038cf>] do_IRQ+0x4f/0xf0
[   10.635490]  [<ffffffff815f8aed>] common_interrupt+0x6d/0x6d
[   10.641805]  <EOI>
[   10.643950]  [<ffffffff8149ca9f>] ? cpuidle_enter_state+0x4f/0xc0
[   10.650972]  [<ffffffff8149ca98>] ? cpuidle_enter_state+0x48/0xc0
[   10.657775]  [<ffffffff8149cb47>] cpuidle_enter+0x17/0x20
[   10.663807]  [<ffffffff810b0070>] cpu_startup_entry+0x2c0/0x3d0
[   10.670423]  [<ffffffff815dfcc7>] rest_init+0x77/0x80
[   10.676065]  [<ffffffff81a60f47>] start_kernel+0x40f/0x41a
[   10.682190]  [<ffffffff81a60941>] ? repair_env_string+0x5c/0x5c
[   10.688799]  [<ffffffff81a60120>] ? early_idt_handlers+0x120/0x120
[   10.695699]  [<ffffffff81a605ee>] x86_64_start_reservations+0x2a/0x2c
[   10.702889]  [<ffffffff81a60733>] x86_64_start_kernel+0x143/0x152
[   10.709689] Code: a0 fc ff 85 c0 8b 4d d4 74 c3 48 8b 7b 08 89 ca 48 c7 c6 60 66 13 a0 31 c0 e8 9d 70 28 e1 8b 4d d4 eb aa 0f 1f 84 00 00 00 00 00 <45> 8b 64 24 3c 48 89 df e8 23 47 4c e1 41 83 fc 01 19 c0 48 83
[   10.731470] RIP  [<ffffffffa0133df0>] ahci_hw_interrupt+0x100/0x130 [libahci]
[   10.739441]  RSP <ffff880033c03d98>
[   10.743333] CR2: 000000000000003c
[   10.747032] ---[ end trace b6e82636970e2690 ]---
[   10.760190] Kernel panic - not syncing: Fatal exception in interrupt
[   10.767291] Kernel Offset: 0x0 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffff9fffffff)

Cc: Alexander Gordeev <agordeev@redhat.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-of-by: David Milburn <dmilburn@redhat.com>
Fixes: 5ca72c4f ("AHCI: Support multiple MSIs")