rt2x00: fix queue timeout checks

Add a timestamp to each queue entry which is updated whenever
the status of the entry changes, and remove the per-queue
timestamps.  The previous check was incorrect and caused both
false positives and false negatives.

With the corrected check it comes apparent that the TX status
usually times out on rt2800usb unless there is sufficient traffic
(i.e. the next TX will complete the previous TX status).

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>