rt2x00: Fix beacon descriptor writing for rt61pci.

The buffer address descriptor word is not part of the TXINFO structure
needed for beacons. The current writing of that word for beacons is
therefore an out-of-bounds write.
Fix this by only writing the buffer address descriptor word for TX
queues.

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>