ath9k: Cleanup beacon queue configuration

Setup the beacon queue parameters after disabling
interrupts. Also, remove the redundant call in conf_tx()
for IBSS mode since the queue would be configured
with the appropriate cwmin/cwmax values when beaconing
is enabled.

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