net: dsa: mv88e6xxx: avoid writing the same mode

There is no need to change the 802.1Q port mode for the same value.
Thus avoid such message:

    [  401.954836] dsa dsa@0 lan0: 802.1Q Mode: Disabled (was Disabled)
Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/mv88e6xxx.c

@@ -1765,16 +1765,21 @@ int mv88e6xxx_port_vlan_filtering(struct dsa_switch *ds, int port,
 
 	old = ret & PORT_CONTROL_2_8021Q_MASK;
 
+	if (new != old) {
 		ret &= ~PORT_CONTROL_2_8021Q_MASK;
 		ret |= new & PORT_CONTROL_2_8021Q_MASK;
 
-	ret = _mv88e6xxx_reg_write(ds, REG_PORT(port), PORT_CONTROL_2, ret);
+		ret = _mv88e6xxx_reg_write(ds, REG_PORT(port), PORT_CONTROL_2,
+					   ret);
 		if (ret < 0)
 			goto unlock;
 
-	netdev_dbg(ds->ports[port], "802.1Q Mode: %s (was %s)\n",
+		netdev_dbg(ds->ports[port], "802.1Q Mode %s (was %s)\n",
 			   mv88e6xxx_port_8021q_mode_names[new],
 			   mv88e6xxx_port_8021q_mode_names[old]);
+	}
+
+	ret = 0;
 unlock:
 	mutex_unlock(&ps->smi_mutex);