ath9k: Fix queue management

Since we use IEEE80211_HW_QUEUE_CONTROL now, the
CAB/Offchannel queues are registered as the last
two queues. There is no need to check and reassign
the queues in the TX start()/done() routines.

CAB frames will not reach the tx() callback since
we set IEEE80211_HW_HOST_BROADCAST_PS_BUFFERING and
pull the buffered frames during beacon transmission.
We also don't have a special HW queue for handling
off-channel frames.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>