rt2x00: Add mesh support

This adds initial support for Mesh Point mode.  For this we tell mac80211 that
we support NL80211_IFTYPE_MESH_POINT.  We also need to send beacons.  mac80211
will configure our RX filter accordingly.

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