ath9k_hw: Fix max rx rate drop for AR9565

Whenever i_coff of IQ calibration is too high, AR9565 drops max
rx rate to MCS4. Skipping IQ update at this time can avoid this
problem for AR9565.

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