ath9k_hw: Cleanup MCI reset routine

* Use a separate function to enable/disable
  OneStepLookAhead.

* Remove unnecessary hardware SREV checks.

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