rt2x00: Implement WDS support

WDS support should be very easy to handle, mac80211 handles
everything for us, so all that is needed is to set the
support flags and handle it in the add_interface() callback.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>