rt2x00: rt2800lib: autodetect 5GHz band support

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>