ath9k: Add Calibration checks

* Prevent divide-by-zero errors in IQ Calibration.
* Do not run temperature compensation if initPDADC or currPDADC is zero.
* Also, introduce a separate function for handling OLC for AR9287.

Signed-off-by: Vivek Natarajan <vnatarajan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>