ath9k: stomp audio profiles on weak signal strength

On lower WLAN signal strength, WLAN downlink traffic might suffer
from retransmissions. At the mean time, playing SCO/A2DP profiles
is affecting WLAN stability. In such scenario, by stomping SCO/A2DP
BT traffic completely for a BTCOEX period, gives WLAN traffic an
oppertunity to recover PHY rate. It also improves WLAN stability at
lower RSSI without sacificing BT traffic.

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