ath9k_hw: run ath9k_hw_9271_pa_cal() initial calibration

The PA calibration for ar9271 ath9k_hw_9271_pa_cal() can run during
reset or initial calibration, update the PA calibration to account
for that and initialize PA calibration variables for both conditions.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>