mac80211: set mcast-rate for mesh point interfaces

mac80211: set mcast-rate for mesh point interfaces

In order to set the multicast rate for mesh point interfaces the "mesh join"
was made explicit and moved to mac80211_setup_vif(), similar to how it is
done for IBSS interfaces.

Previously, the mesh join was made implicit in case authentication (i.e.
$key) was not used when creating the interface in mac80211_prepare_vif(),
while using authentication would create the interface first, then join
later in mac80211_setup_vif() by starting authsae.

Signed-off-by: Nils Schneider <nils@nilsschneider.net>
Backport of r47408

git-svn-id: svn://svn.openwrt.org/openwrt/branches/chaos_calmer@48858 3c298f89-4303-0410-b956-a3cf2f4a3e73