ath9k_hw: Fix concurrent tx on lower tx power

Whenever WLAN receives scheduling msg from BT, it reduces tx power
based on RSSI level. And then BT starts simultaneous transmission
along with WLAN. Sometimes HW MAC compares tx power that is used
prior to power reduction which is causing BT transmission to defer.

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