b43legacy: Fix crash on unload when firmware not available

When b43legacy is loaded without the firmware being available, a following
unload generates a kernel NULL pointer dereference BUG as follows:

[  214.330789] BUG: unable to handle kernel NULL pointer dereference at 0000004c
[  214.330997] IP: [<c104c395>] drain_workqueue+0x15/0x170
[  214.331179] *pde = 00000000
[  214.331311] Oops: 0000 [#1] SMP
[  214.331471] Modules linked in: b43legacy(-) ssb pcmcia mac80211 cfg80211 af_packet mperf arc4 ppdev sr_mod cdrom sg shpchp yenta_socket pcmcia_rsrc pci_hotplug pcmcia_core battery parport_pc parport floppy container ac button edd autofs4 ohci_hcd ehci_hcd usbcore usb_common thermal processor scsi_dh_rdac scsi_dh_hp_sw scsi_dh_emc scsi_dh_alua scsi_dh fan thermal_sys hwmon ata_generic pata_ali libata [last unloaded: cfg80211]
[  214.333421] Pid: 3639, comm: modprobe Not tainted 3.6.0-rc6-wl+ #163 Source Technology VIC 9921/ALI Based Notebook
[  214.333580] EIP: 0060:[<c104c395>] EFLAGS: 00010246 CPU: 0
[  214.333687] EIP is at drain_workqueue+0x15/0x170
[  214.333788] EAX: c162ac40 EBX: cdfb8360 ECX: 0000002a EDX: 00002a2a
[  214.333890] ESI: 00000000 EDI: 00000000 EBP: cd767e7c ESP: cd767e5c
[  214.333957]  DS: 007b ES: 007b FS: 00d8 GS: 0033 SS: 0068
[  214.333957] CR0: 8005003b CR2: 0000004c CR3: 0c96a000 CR4: 00000090
[  214.333957] DR0: 00000000 DR1: 00000000 DR2: 00000000 DR3: 00000000
[  214.333957] DR6: ffff0ff0 DR7: 00000400
[  214.333957] Process modprobe (pid: 3639, ti=cd766000 task=cf802e90 task.ti=cd766000)
[  214.333957] Stack:
[  214.333957]  00000292 cd767e74 c12c5e09 00000296 00000296 cdfb8360 cdfb9220 00000000
[  214.333957]  cd767e90 c104c4fd cdfb8360 cdfb9220 cd682800 cd767ea4 d0c10184 cd682800
[  214.333957]  cd767ea4 cba31064 cd767eb8 d0867908 cba31064 d087e09c cd96f034 cd767ec4
[  214.333957] Call Trace:
[  214.333957]  [<c12c5e09>] ? skb_dequeue+0x49/0x60
[  214.333957]  [<c104c4fd>] destroy_workqueue+0xd/0x150
[  214.333957]  [<d0c10184>] ieee80211_unregister_hw+0xc4/0x100 [mac80211]
[  214.333957]  [<d0867908>] b43legacy_remove+0x78/0x80 [b43legacy]
[  214.333957]  [<d083654d>] ssb_device_remove+0x1d/0x30 [ssb]
[  214.333957]  [<c126f15a>] __device_release_driver+0x5a/0xb0
[  214.333957]  [<c126fb07>] driver_detach+0x87/0x90
[  214.333957]  [<c126ef4c>] bus_remove_driver+0x6c/0xe0
[  214.333957]  [<c1270120>] driver_unregister+0x40/0x70
[  214.333957]  [<d083686b>] ssb_driver_unregister+0xb/0x10 [ssb]
[  214.333957]  [<d087c488>] b43legacy_exit+0xd/0xf [b43legacy]
[  214.333957]  [<c1089dde>] sys_delete_module+0x14e/0x2b0
[  214.333957]  [<c110a4a7>] ? vfs_write+0xf7/0x150
[  214.333957]  [<c1240050>] ? tty_write_lock+0x50/0x50
[  214.333957]  [<c110a6f8>] ? sys_write+0x38/0x70
[  214.333957]  [<c1397c55>] syscall_call+0x7/0xb
[  214.333957] Code: bc 27 00 00 00 00 a1 74 61 56 c1 55 89 e5 e8 a3 fc ff ff 5d c3 90 55 89 e5 57 56 89 c6 53 b8 40 ac 62 c1 83 ec 14 e8 bb b7 34 00 <8b> 46 4c 8d 50 01 85 c0 89 56 4c 75 03 83 0e 40 80 05 40 ac 62
[  214.333957] EIP: [<c104c395>] drain_workqueue+0x15/0x170 SS:ESP 0068:cd767e5c
[  214.333957] CR2: 000000000000004c
[  214.341110] ---[ end trace c7e90ec026d875a6 ]---Index: wireless-testing/drivers/net/wireless/b43legacy/main.c

The problem is fixed by making certain that the ucode pointer is not NULL
before deregistering the driver in mac80211.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Stable <stable@vger.kernel.org>   [v 3.3.0+]
Signed-off-by: John W. Linville <linville@tuxdriver.com>