ath9k: improve BT FTP/PAN performance

When BT FTP/PAN transmits while WLAN is idle, the one of 9462 chain
often picks up BT's tx signal and starts receiving. If the current
weight is set to be higher than BT tx, BT tx will be aborted and this
also degrades BT performance. Hence lower WLAN rx priority in this
case only when there are no WLAN traffic.

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