rt2x00: Include ATIM queue support in rt2x00queue_get_tx_queue.

The ATIM queue is considered to be a TX queue by the drivers that support
the queue. Therefore include support for the ATIM queue to the
rt2x00queue_get_tx_queue function so that the drivers that support the ATIM
queue can also use that function.

Add the support in such a way that drivers that do not support the ATIM
queue are not penalized in their efficiency.

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>