ath9k: Cleanup calibration interface

This patch cleans up the functions dealing with calibration,
using proper return values.
ath9k_hw_per_calibration(), ath9k_hw_calibrate now return bool values
instead of setting error values in the function arguments.

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