ath9k: Fix PeakDetect calibration for AR9462

Since HW PeakDetect calibration is turned on for AR9462,
various conditions have to be handled in the driver:

* Enable agc_cal when loading RTT fails.
* Disable SW PeakDetect calibration when RTT calibration is not enabled.
* Keep SW PeakDetect calibration result in driver.
* Update RTT table according to the saved value.
* Write RTT back after modifying SW RTT table.
* Enable local mode for PeakDetect calibration and restore values.

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