mac80211/rt2x00: allow disabling bands through dts

Add dts property ralink,2ghz and ralink,5ghz
e.g. disable 2ghz band:
wmac@10180000 {
  status = "okay";
  ralink,2ghz = <0>;
}

Signed-off-by: Alexander Couzens <lynxis@fe80.eu>

SVN-Revision: 42281