ath9k_hw: Fix throughput drops in HT40 mode for AR9287 chips

Doing adc gain calibration for AR9287 chips is causing
throughput drops in HT40 mode. Remove ADC Gain from supported
calibration list.

Signed-off-by: Rajkumar Manoharan <rmanoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>