ath9k: Fix bug in calibration initialization

This patch fixes a bug in ath9k_hw_init_cal() where the wrong
calibration was being done for non-AR9285 chipsets.
Also add a few helpful comments.

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