ath9k: Fix chainmask selection for AR9462

Force chain 1 to be used for CCK rates since the target power
table stored in EEPROM is too high to transmit with both chains.
This is needed to avoid regulatory violation.

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