ar9170usb: fix hang on resume

This patch fixes a hang on resume when the filesystem is not
available and request_firmware blocks.

However, the device does not accept the firmware on resume.
and it will exit with:

> firmware part 1 upload failed (-71).
> device is in a bad state. please reconnect it!

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