ath9k: adjust WLAN and BT concurrent transmission

The simulataneous transmission of both WLAN and BT might cause
increase in power levels. To avoid regulatory violation, WLAN tx
power will be adjusted according to BT power index based on avaliability
of BT scheduling messages. WLAN tx power reduction might affect its
performance. So WLAN tx power is only be lowered when the signal strength
is good enough. Otherwise concurrent tx will be disabled and WLAN uses
it default power levels. Also concurrent tx is disabled whenever WLAN is
moving to off-channel which might be used by BT.

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