mac80211_hwsim: Fix bcn_en_iter to use atomic iteration

'mac80211_hwsim: Fix tracking of beaconing for multi-vif' introduced an
iteration of active interfaces into the bss_info_changed handler.
However, it used a wrong type of iteration and could result in a dead
lock since iflist_mtx can already be held. Fix this by using the atomic
version of the iteration function.

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