net: export __dev_addr_sync/__dev_addr_unsync

For mac80211, with the master netdev removal, we need to be
able to sync a multicast address list onto another list that
is not tracked within a netdev, so we need access to the
functions doing that.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>