ath9k: allow disabling bands via platform data

Some devices have multiple bands enables in the EEPROM data, even though
they are only calibrated for one. Allow platform data to disable
unsupported bands.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>