ath9k_hw: Fix radio retention for AR9462

IQ calibration during fast channel change sometimes failed
with RTT. And also restoring invalid radio retention readings
during init cal could cause failure to set the channel properly.
This patch counts the valid rtt history readings and clears
rtt mask.

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