cfg80211: fix oops due to unassigned set_monitor_enabled callback

Quick fix for method being invoked without checking its existence.
Signed-off-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/wireless/core.c

@@ -774,6 +774,7 @@ void cfg80211_update_iface_num(struct cfg80211_registered_device *rdev,
 
 	has_monitors_only_new = cfg80211_has_monitors_only(rdev);
 	if (has_monitors_only_new != has_monitors_only_old) {
+		if (rdev->ops->set_monitor_enabled)
 			rdev->ops->set_monitor_enabled(&rdev->wiphy,
 						       has_monitors_only_new);