ath9k: Fix antenna diversity for CUS198

CUS198/CUS230 need a few tweaks in the antenna diversity
algorithm to accomodate RSSI variation. Add a couple
of knobs to control low RSSI threshold and fast antenna
diversity bias values.

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