mac80211_hwsim: Fix tracking of beaconing for multi-vif

mac80211_hwsim canceled beacon_timer on any vif changing from enabled
to disabled beaconing. This breaks cases where there are multiple
beaconing vifs and only one of them is removed. Fix this by tracking
beaconing status per vif and disable beacon_timer only if no active vif
remain with beaconing enabled.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>