ath9k: clear bb filter calibration power threshold

JP WiFi certification for bandwidth of channel 14 failed, the OBW
is lower than the requirement. Clear the bb filter calibration power
threshold to increase OBW(+2). The fix only for qca9531 chip now.

Signed-off-by: Miaoqing Pan <miaoqing@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>