ath9k_hw: Cleanup TX power calculation for AR9287

* Add a few comments, and move the updation of max_power_level
  to a helper routine. This is also done by non-4K based chipsets,
  this will be fixed in a separate patch.

* Remove two WARs which are required for old AR5416 chipsets,
  and are not needed for AR9287.

* Fix indentation and make things readable.

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