ath9k_hw: Fix magnitude/phase average in TxIQ Calibration

The commit "ath9k_hw: Fix Tx IQ Calibration hang issue in
AR9003 chips" did not consider more than one potential sample
while calculating magnitude/phase average if more than one
sample has the same value which could affect post-processing
of outlier detection that causes an undesirable Tx IQ
correction value will be assigned to tx gain settings where
outlier happens.

Cc: Kai Shi <kaishi@qca.qualcomm.com>
Reported-by: Paul Stewart <pstew@google.com>
Signed-off-by: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Signed-off-by: Paul Stewart <pstew@google.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>