net: dsa: sja1105: increase MTU to account for VLAN header on DSA ports

Since all packets are transmitted as VLAN-tagged over a DSA link (this
VLAN tag represents the tag_8021q header), we need to increase the MTU
of these interfaces to account for the possibility that we are already
transporting a user-visible VLAN header.
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/sja1105/sja1105_main.c

@@ -982,7 +982,7 @@ static int sja1105_init_l2_policing(struct sja1105_private *priv)
 	for (port = 0; port < ds->num_ports; port++) {
 		int mtu = VLAN_ETH_FRAME_LEN + ETH_FCS_LEN;
 
-		if (dsa_is_cpu_port(priv->ds, port))
+		if (dsa_is_cpu_port(ds, port) || dsa_is_dsa_port(ds, port))
 			mtu += VLAN_HLEN;
 
 		policing[port].smax = 65535; /* Burst size in bytes */
@@ -2664,7 +2664,7 @@ static int sja1105_change_mtu(struct dsa_switch *ds, int port, int new_mtu)
 
 	new_mtu += VLAN_ETH_HLEN + ETH_FCS_LEN;
 
-	if (dsa_is_cpu_port(ds, port))
+	if (dsa_is_cpu_port(ds, port) || dsa_is_dsa_port(ds, port))
 		new_mtu += VLAN_HLEN;
 
 	policing = priv->static_config.tables[BLK_IDX_L2_POLICING].entries;