rt2x00: Clean up all driver's kick_tx_queue callback functions.

All of the driver's kick_tx_queue callback functions treat the TX queue
for beacons in a special manner.
Clean this up by integrating the kicking of the beacon queue into the
write_beacon callback function, and let the generic code no longer call
the kick_tx_queue callback function when updating the beacon.

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