ar9170usb: fix read from freed driver context

Commit "ar9170: wait for asynchronous firmware loading"
introduced a bug, which is triggered by fatal errors
while the driver is initializing the device.

BUG: unable to handle kernel paging request at 6b6b6bf7
IP: [<c117b567>] kobject_put+0x7/0x70
*pde = 00000000
Oops: 0000 [#1] PREEMPT
last sysfs file: /sys/devices/platform/hdaps/position
Modules linked in: ar9170usb [...]

Pid: 6246, comm: firmware/ar9170 Not tainted 2.6.34-wl #54
EIP: 0060:[<c117b567>] EFLAGS: 00010206 CPU: 0
EIP is at kobject_put+0x7/0x70
EAX: 6b6b6bd7 EBX: f4d3d0e0 ECX: f5ba9124 EDX: f6af2a7c
ESI: 00000000 EDI: f4d3d0e0 EBP: 00000000 ESP: f5e98f9c
 DS: 007b ES: 007b FS: 0000 GS: 0000 SS: 0068
Process firmware/ar9170 (pid: 6246)
Stack:
 c12532ed 00000246 f5bfaa70 f8487353 f4d3d0e0
Call Trace:
 [<c12532ed>] ? device_release_driver+0x1d/0x30
 [<f8487353>] ? ar9170_usb_firmware_failed+0x43/0x70 [ar9170usb]
 [<c125983c>] ? request_firmware_work_func+0x2c/0x70
 [<c1259810>] ? request_firmware_work_func+0x0/0x70
 [<c10413f4>] ? kthread+0x74/0x80
 [<c1041380>] ? kthread+0x0/0x80
 [<c1003136>] ? kernel_thread_helper+0x6/0x10
Code: 40 d3 f2 ff 85 c0 89 c3 74 0a ba 44 86 4c c1 e8 [...]
EIP: [<c117b567>] kobject_put+0x7/0x70 SS:ESP 0068:f5e98f9c
CR2: 000000006b6b6bf7
---[ end trace e81abb992434b410 ]---

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>