mac80211: keep mesh ifaces in allmulti mode

Currently a mesh node will not forward a multicast frame if it is not subscribed
to the specific multicast address. This patch addresses the issue and fixes mesh
multicast forwarding.

Signed-off-by: Luis Carlos Cobo <luisca@cozybit.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>