mac80211: check add_chanctx callback before use in ieee80211_reconfig

During testing our mac80211 driver a fatal error occurred which was
signalled to mac80211. Upon performing the reconfiguration of the
device a WARN_ON was triggered. This warning checked the return value
of drv_add_chanctx(). However, this returns -EOPNOTSUPP when the
driver does not provide the callback. As the callback is optional
better check it is defined before calling drv_add_chanctx().
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/util.c

@@ -1417,10 +1417,12 @@ int ieee80211_reconfig(struct ieee80211_local *local)
 	}
 
 	/* add channel contexts */
+	if (local->use_chanctx) {
 		mutex_lock(&local->chanctx_mtx);
 		list_for_each_entry(ctx, &local->chanctx_list, list)
 			WARN_ON(drv_add_chanctx(local, ctx));
 		mutex_unlock(&local->chanctx_mtx);
+	}
 
 	list_for_each_entry(sdata, &local->interfaces, list) {
 		struct ieee80211_chanctx_conf *ctx_conf;