ath9k: Calculate IQ-CAL median

This patch adds a routine to calculate the median IQ correction
values for AR955x, which is used for outlier detection.
The normal method which is used for all other chips is
bypassed for AR955x.

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