net: dsa: mv88e6xxx: fix uninitialized error return

The error return err is not initialized and there is a possibility
that err is not assigned causing mv88e6xxx_port_bridge_join to
return a garbage error return status. Fix this by initializing err
to 0.
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Reviewed-by: NVivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/dsa/mv88e6xxx.c

@@ -2181,7 +2181,7 @@ int mv88e6xxx_port_bridge_join(struct dsa_switch *ds, int port,
 			       struct net_device *bridge)
 {
 	struct mv88e6xxx_priv_state *ps = ds_to_priv(ds);
-	int i, err;
+	int i, err = 0;
 
 	mutex_lock(&ps->smi_mutex);