ath9k_hw: add default chainmask for AR9462

the default tx/rx chainmask for AR9462 is 0x3.
this patch helps to assign 0x3 rather than 0x7 for
AR9462 with the help of fix_chainmask module if something
goes wrong in reading tx/rx chain mask from OTP/EEPROM
card(though its very unlikely)

Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>