rt2x00: Clean up generic procedures on descriptor writing.

With a little bit of restructuring it isn't necessary to have special
cases in rt2x00queue_write_tx_descriptor for writing the descriptor
for beacons.
Simply split off the kicking of the TX queue to a separate function
with is only called for non-beacons.

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>