ath9k: Modify IQ calibration for AR955x

IQ calibration post-processing for AR955x is different
from other chips - instead of just doing it as part
of AGC calibration once, it is triggered 3 times and
a median is determined. This patch adds initial support
for changing the calibration behavior for AR955x.

Also, to simplify things, a helper routine to issue/poll
AGC calibration is used.

For non-AR955x chips, the iqcal_idx (which will be used
in subsequent patches) is set to zero.

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