ath5k: disable 5 GHz support for the dualband PHY chip on dual-radio AR5312

There are two variants of AR5312 dual-band devices, one single-radio
and the other one dual-radio. On the dual-radio board, the first MAC
only supports 5 GHz, even though it has a dual-band PHY. The 2.4 GHz
part of this phy is used in pass-through mode, connecting the second
MAC with the second PHY.
Disable 2.4 GHz for the first MAC on an AR5312, but only if the board
configuration indicates a dual-radio device.

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>