If the RF chip supports more than 14 channels that
indirectly means that it supports the 5GHz band.
Use this fact to enable 5GHz band support instead
of setting SUPPORT_BAND_5GHZ separately for each
RF chip.

Also move the setup code of the 2GHz band to the
same place.
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>