rt2x00: Move watchdog work to kernel work_queue

The watchdog function must run on a work_queue
which is independent of any other work inside rt2x00.

The main reasons, being that a broken work on the mac80211
work_queue can otherwise prevent the watchdog to run (while
in fact the watchdog could fix the issue). And on the other
hand because the watchdog relies on the completion of the
completion handlers for RX/TX which for the USB case, occur
on the mac80211 workqueue.

This fixes some "Queue %d failed to flush" errors, which were
caused by the watchdog function waiting on the completion
handler which was scheduled to run right after the watchdog.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Acked-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>