net: dsa: properly check for the bridge_leave methods in dsa_switch_bridge_leave()

This was not caught because there is no switch driver which implements
the .port_bridge_join but not .port_bridge_leave method, but it should
nonetheless be fixed, as in certain conditions (driver development) it
might lead to NULL pointer dereference.

Fixes: f66a6a69 ("net: dsa: permit cross-chip bridging between all trees in the system")
Signed-off-by: NVladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/dsa/switch.c

@@ -113,11 +113,11 @@ static int dsa_switch_bridge_leave(struct dsa_switch *ds,
 	int err, port;
 
 	if (dst->index == info->tree_index && ds->index == info->sw_index &&
-	    ds->ops->port_bridge_join)
+	    ds->ops->port_bridge_leave)
 		ds->ops->port_bridge_leave(ds, info->port, info->br);
 
 	if ((dst->index != info->tree_index || ds->index != info->sw_index) &&
-	    ds->ops->crosschip_bridge_join)
+	    ds->ops->crosschip_bridge_leave)
 		ds->ops->crosschip_bridge_leave(ds, info->tree_index,
 						info->sw_index, info->port,
 						info->br);