ath9k_hw: Do fast channel change based on reusable calibration results

Support the fast channel change across band switch only when there
are available of reusable cabliration results. And also observed that
doing agc control calibration on fastcc, sometimes causing calibration
timeout. Hence changing agc control to be run only on full chip reset.

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