rt2x00: Correctly kill beacon queue

USB devices upload their beacon and then automatically send
it out every beacon interval. However when killing a TX queue
we only kill the URB and not the actual transmission of the beacon.

This will reset the Beacon register to prevent any beacons from
being transmitted.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>