mac80211: fix rx monitor filter refcounters

This patch fixes an refcounting bug. Previously it
was possible to corrupt the per-device recv. filter
and monitor management counters when:
	iw dev wlanX set monitor [new flags]
was issued on an active monitor interface.

Acked-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>