rt2800usb: handle TX status timeouts

The watchdog just triggers rt2800usb_work_txdone() when it
detects a TX status timeout, thus rt2800usb_work_txdone() needs to
handle this case.

Signed-off-by: Johannes Stezenbach <js@sig21.net>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>