ath9k_hw: fix IQ calibration chain index

The chain index to perform IQ calibration is counted to number of valid
tx chains and then used for indexing chain specific registers. If the
chainmask is set to 0x2 (i.e chain 1 only), still it accesses chain 0
registers for chain 1. So use real chain index instead sequential one.

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