ath9k: Fix calibration for AR9462

TX IQ calibration is disabled by default for AR9462, this
is done using the initvals (reg 0xa644).

But, to compensate for this, the AR_PHY_RX_DELAY register
should be set to the max allowed value when performing
calibration.

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