ath9k: enable 2GHz band only if the device supports it

Currently, the 2GHz band is enabled unconditionally, even if the device
does not support it.

Changes-licensed-under: ISC
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>