mac80211: improve cleanup when leaving mesh

It is not necessary to stop the mesh beacon in the mac80211 ndo_stop
handler, since cfg80211 has already left the mesh on NETDEV_GOING_DOWN
notification.

Also some improvements to ieee80211_stop_mesh():

 - flush mpath entries.
 - flush sta entries per-sdata so we don't remove entries belonging to
   other vifs on the same hw.

Signed-off-by: Thomas Pedersen <thomas@cozybit.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>