rt2x00: Allow rt61 to catch up after a missing tx report

Sometimes it happens in the tx path that an entry given to the hardware isn't
reported in the txdone handler. This ultimately led to the dreaded "non-free
entry in the non-full queue" message and the stopping of the tx queue. Work
around this issue by allowing the driver to also clear out previos entries in
the txdone handler.

Signed-off-by: Mattias Nissler <mattias.nissler@gmx.de>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>