mac80211: use common cleanup for user/!user_mpm

We've accumulated a couple of different fixes now to mesh_sta_cleanup()
due to the different paths that user_mpm and !user_mpm cases take -- one
fix to flush nexthop paths and one to fix the counting.

The only caller of mesh_plink_deactivate() is mesh_sta_cleanup(), so we
can push the user_mpm checks down into there in order to share more
code.

In doing so, we can remove an extra call to mesh_path_flush_by_nexthop()
and the (unnecessary) call to mesh_accept_plinks_update().  This will
also ensure the powersaving state code gets called in the user_mpm case.

The only cleanup tasks we need to avoid when MPM is in user-space
are sending the peering frames and stopping the plink timer, so wrap
those in the appropriate check.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>