ath9k_hw: Fix calibration for AR9287 devices

AR9287 based devices have issues with ADC gain calibration
which would cause uplink throughput drops in HT40 mode.
Remove ADC gain from the supported calibration algorithms.

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