ath5k: disable 5 GHz support if a 2.4 GHz radio is detected

On a dual-radio dual-band AR5312 device, the calibration data is shared
between the 5 GHz and the 2.4 GHz radio/MAC.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>