mac80211: move mesh filter adjusting

Logically, the filter adjusting belongs with
starting/stopping mesh, not interface up/down,
so move it there.

Tested-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>