rt2x00: Optimize calls to rt2x00queue_get_queue

In some cases (tx path for example) we don't need to check for non-tx
queues in rt2x00queue_get_queue. Hence, introduce a new method
rt2x00queue_get_tx_queue that is only valid for tx queues and use it in
places where only tx queues are valid.

Furthermore, this new method is quite short and as such can be inlined
to avoid the function call overhead.

This only converts the txdone functions of drivers that don't use an ATIM
queue and the generic tx path.

Signed-off-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>