ath9k: Fix bug in determining calibration support

ADC gain calibration has to be done for all non 2GHZ-HT20 channels.
Regression from "ath9k: use ieee80211_conf on ath9k_hw_iscal_supported()"

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