ath9k_hw: Do not enable IQ-CAL for half/quarter rates

IQ calibration doesn't complete and times out for half/quarter
rates, so skip it correctly.

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