rt2x00: Optimize getting the beacon queue structure.

In the spirit of optimizing the code to get the queue structure of TX queues,
also optimize the code to get beacon queues. We can simply use the bcn queue
field of the rt2x00_dev structure instead of using the rt2x00queue_get_queue
function.

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