mac80211: Learn about mesh portals from multicast traffic

Mesh portals proxy traffic for nodes external to the mesh.  When a
proxied frame is received by a mesh interface, it should update its mesh
portal table.  This was only happening for unicast frames.  With this
change we also learn about mesh portals from proxied multicast frames.

Signed-off-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>