mac80211: optimise AP stop RCU handling

If there are VLANs, stopping an AP is inefficient as it
calls rcu_barrier() once for each interface (the VLANs
and the AP itself). Optimise this by moving rcu_barrier()
out of the station cleanups and calling it only once for
all interfaces combined.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>