ar9170: wait for asynchronous firmware loading

This patch fixes a regression introduced by the following patch:
"ar9170: load firmware asynchronously"

When we kick off a firmware loading request and then unbind,
or disconnect the usb device right away, we get into trouble:

> ------------[ cut here ]------------
> WARNING: at lib/kref.c:44 kref_get+0x1c/0x20()
> Hardware name: 18666GU
> Modules linked in: ar9170usb [...]
> Pid: 6588, comm: firmware/ar9170 Not tainted 2.6.34-rc5-wl #43
> Call Trace:
> [<c102b05e>] ? warn_slowpath_common+0x6e/0xb0
> [<c117c93c>] ? kref_get+0x1c/0x20
> [<c102b0b3>] ? warn_slowpath_null+0x13/0x20
> [<c117c93c>] ? kref_get+0x1c/0x20
> [<c117bb2f>] ? kobject_get+0xf/0x20
> [<c124d630>] ? get_device+0x10/0x20
> [<c124e5a0>] ? device_add+0x60/0x530
> [<c117b8b5>] ? kobject_init+0x25/0xa0
> [<c12569f9>] ? _request_firmware+0x139/0x3e0
> [<c1256cc0>] ? request_firmware_work_func+0x20/0x70
> [<c1256ca0>] ? request_firmware_work_func+0x0/0x70
> [<c103ff24>] ? kthread+0x74/0x80
> [<c103feb0>] ? kthread+0x0/0x80
> [<c1003136>] ? kernel_thread_helper+0x6/0x10
>---[ end trace 2d50bd818f64a1b7 ]---
- followed by a random Oops -

Avoid that by waiting for the firmware loading to finish
(whether successfully or not) before the unbind in
ar9170_usb_disconnect.

Reported-by: Johannes Berg <johannes@sipsolutions.net>
Bug-fixed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>