usb: musb: omap2430: fix the readiness check in omap_musb_mailbox

The check in omap_musb_mailbox does not properly check if the module has
been fully initialized. The patch fixes that, and the kernel panic below:

$ modprobe twl4030-usb
[   13.924743] twl4030_usb twl4030-usb.33: HW_CONDITIONS 0xe0/224; link 3
[   13.940307] Unable to handle kernel NULL pointer dereference at virtual address 00000004
[   13.948883] pgd = ef27c000
[   13.951751] [00000004] *pgd=af256831, *pte=00000000, *ppte=00000000
[   13.958374] Internal error: Oops: 17 [#1] ARM
[   13.962921] Modules linked in: twl4030_usb(+) omap2430 libcomposite
[   13.969543] CPU: 0    Not tainted  (3.8.0-rc1-n9xx-11758-ge37a37c-dirty #6)
[   13.976867] PC is at omap_musb_mailbox+0x18/0x54 [omap2430]
[   13.982727] LR is at twl4030_usb_probe+0x240/0x354 [twl4030_usb]
[   13.989013] pc : [<bf013b6c>]    lr : [<bf018958>]    psr: 60000013
[   13.989013] sp : ef273cf0  ip : ef273d08  fp : ef273d04
[   14.001068] r10: bf01b000  r9 : bf0191d8  r8 : 00000001
[   14.006530] r7 : 00000000  r6 : ef140e10  r5 : 00000003  r4 : 00000000
[   14.013397] r3 : bf0142dc  r2 : 00000006  r1 : 00000000  r0 : 00000003
[   14.020233] Flags: nZCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment user
[   14.027740] Control: 10c5387d  Table: af27c019  DAC: 00000015
[   14.033752] Process modprobe (pid: 616, stack limit = 0xef272238)
[   14.040161] Stack: (0xef273cf0 to 0xef274000)
[   14.044708] 3ce0:                                     ef254310 00000001 ef273d34 ef273d08
[   14.053314] 3d00: bf018958 bf013b60 bf0190a4 ef254310 c0101550 c0c3a138 ef140e10 ef140e44
[   14.061889] 3d20: bf019150 00000001 ef273d44 ef273d38 c019890c bf018724 ef273d64 ef273d48
[   14.070495] 3d40: c01974fc c01988f8 ef140e10 bf019150 ef140e44 00000000 ef273d84 ef273d68
[   14.079071] 3d60: c0197728 c019748c c0197694 00000000 bf019150 c0197694 ef273dac ef273d88
[   14.087677] 3d80: c0195c38 c01976a0 ef03610c ef143eb0 c0128954 ef254780 bf019150 c0b19548
[   14.096252] 3da0: ef273dbc ef273db0 c0197098 c0195bf0 ef273dec ef273dc0 c0196c98 c0197080
[   14.104858] 3dc0: bf0190a4 c0b27bc0 ef273dec bf019150 bf019190 c0b27bc0 ef272000 00000001
[   14.113433] 3de0: ef273e14 ef273df0 c0197c18 c0196b30 ef273f48 bf019190 c0b27bc0 ef272000
[   14.122039] 3e00: 00000001 bf01b000 ef273e24 ef273e18 c0198b28 c0197ba4 ef273e34 ef273e28
[   14.130615] 3e20: bf01b014 c0198ae8 ef273e8c ef273e38 c0008918 bf01b00c c004f730 c012ba1c
[   14.139221] 3e40: ef273e74 00000000 c00505b0 c004f72c 00000000 ef273e60 ef273f48 bf019190
[   14.147796] 3e60: 00000001 ef273f48 bf019190 00000001 ef286340 00000001 bf0191d8 c0065414
[   14.156402] 3e80: ef273f44 ef273e90 c0067754 c00087fc bf01919c 00007fff c0064794 00000000
[   14.164978] 3ea0: ef273ecc f0064000 00000001 ef272000 ef272000 00067f39 bf0192b0 bf01919c
[   14.173583] 3ec0: ef273f0c ef273ed0 c00a6bf0 c00a53fc ff000000 000000d2 c0067dc8 00000000
[   14.182159] 3ee0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
[   14.190765] 3f00: 00000000 00000000 00000000 00000000 00000000 00000000 ffffffff 00002968
[   14.199340] 3f20: 00080878 00067f39 00000080 c000e2e8 ef272000 00000000 ef273fa4 ef273f48
[   14.207946] 3f40: c0067e54 c0066188 f0064000 00002968 f0065530 f0065463 f0065fb0 000012c4
[   14.216522] 3f60: 00001664 00000000 00000000 00000000 00000014 00000015 0000000c 00000000
[   14.225128] 3f80: 00000008 00000000 00000000 00080370 00080878 0007422c 00000000 ef273fa8
[   14.233703] 3fa0: c000e140 c0067d80 00080370 00080878 00080878 00002968 00067f39 00000000
[   14.242309] 3fc0: 00080370 00080878 0007422c 00000080 00074030 00067f39 bec7aef8 00000000
[   14.250885] 3fe0: b6f05300 bec7ab68 0000e93c b6f05310 60000010 00080878 af7fe821 af7fec21
[   14.259460] Backtrace:
[   14.262054] [<bf013b54>] (omap_musb_mailbox+0x0/0x54 [omap2430]) from [<bf018958>] (twl4030_usb_probe+0x240/0x354 [twl4030_usb])
[   14.274200]  r5:00000001 r4:ef254310
[   14.277984] [<bf018718>] (twl4030_usb_probe+0x0/0x354 [twl4030_usb]) from [<c019890c>] (platform_drv_probe+0x20/0x24)
[   14.289123]  r8:00000001 r7:bf019150 r6:ef140e44 r5:ef140e10 r4:c0c3a138
[   14.296203] [<c01988ec>] (platform_drv_probe+0x0/0x24) from [<c01974fc>] (driver_probe_device+0x7c/0x214)
[   14.306243] [<c0197480>] (driver_probe_device+0x0/0x214) from [<c0197728>] (__driver_attach+0x94/0x98)
[   14.316009]  r7:00000000 r6:ef140e44 r5:bf019150 r4:ef140e10
[   14.321990] [<c0197694>] (__driver_attach+0x0/0x98) from [<c0195c38>] (bus_for_each_dev+0x54/0x88)
[   14.331390]  r6:c0197694 r5:bf019150 r4:00000000 r3:c0197694
[   14.337371] [<c0195be4>] (bus_for_each_dev+0x0/0x88) from [<c0197098>] (driver_attach+0x24/0x28)
[   14.346588]  r6:c0b19548 r5:bf019150 r4:ef254780
[   14.351440] [<c0197074>] (driver_attach+0x0/0x28) from [<c0196c98>] (bus_add_driver+0x174/0x244)
[   14.360687] [<c0196b24>] (bus_add_driver+0x0/0x244) from [<c0197c18>] (driver_register+0x80/0x154)
[   14.370086]  r8:00000001 r7:ef272000 r6:c0b27bc0 r5:bf019190 r4:bf019150
[   14.377136] [<c0197b98>] (driver_register+0x0/0x154) from [<c0198b28>] (platform_driver_register+0x4c/0x60)
[   14.387390] [<c0198adc>] (platform_driver_register+0x0/0x60) from [<bf01b014>] (twl4030_usb_init+0x14/0x1c [twl4030_usb])
[   14.398895] [<bf01b000>] (twl4030_usb_init+0x0/0x1c [twl4030_usb]) from [<c0008918>] (do_one_initcall+0x128/0x1a8)
[   14.409790] [<c00087f0>] (do_one_initcall+0x0/0x1a8) from [<c0067754>] (load_module+0x15d8/0x1bf8)
[   14.419189] [<c006617c>] (load_module+0x0/0x1bf8) from [<c0067e54>] (sys_init_module+0xe0/0xf4)
[   14.428344] [<c0067d74>] (sys_init_module+0x0/0xf4) from [<c000e140>] (ret_fast_syscall+0x0/0x30)
[   14.437652]  r6:0007422c r5:00080878 r4:00080370
[   14.442504] Code: e24cb004 e59f3038 e1a05000 e593401c (e5940004)
[   14.448944] ---[ end trace dbf47e5bc5ba03c2 ]---
[   14.453826] Kernel panic - not syncing: Fatal exception

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Felipe Balbi <balbi@ti.com>