mac80211: refactor virtual monitor code

Use cfg80211 the new .set_monitor_enabled instead
of tracking it inside mac80211.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

diff --cc net/mac80211/cfg.c
Simple merge

diff --cc net/mac80211/ieee80211_i.h
Simple merge

diff --cc net/mac80211/iface.c
Simple merge