ath9k_hw: improve ANI processing and rx desensitizing parameters

This patch improves ANI operations by switching among the immunity
levels based on PHY errors and beacon rssi which will adjust receiver
desensitizing parameters. The changes are

* Configure the Weak Signal Detection based on current immunity value.
* At highest OFDM immunity level poor performance was observed with
  strong interference. By tuning the FIR step and spur immunity levels
  and not changing any weak signal detection thresholds at any level
  helped to improve the performance.
* ANI took long time to recover back to lower immunity levels on heavy
  data load. As the listen time got reset to zero before reaching to
  the 5x of aniperiod, the immunity level is not lowering back even
  without any interference. This patch fix that.

Cc: Paul Stewart <pstew@google.com>
Cc: Susinder Gulasekaran <susinder@qca.qualcomm.com>
Signed-off-by: Suresh Chandrasekaran <csuresh@qca.qualcomm.com>
Signed-off-by: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>