net: ethernet: mtk_eth_soc: fix MTU warnings

in recent kernel versions there are warnings about incorrect MTU size
like these:

eth0: mtu greater than device maximum
mtk_soc_eth 1b100000.ethernet eth0: error -22 setting MTU to include DSA overhead

Fixes: bfcb8132 ("net: dsa: configure the MTU for switch ports")
Fixes: 72579e14 ("net: dsa: don't fail to probe if we couldn't set the MTU")
Fixes: 7a4c53be ("net: report invalid mtu value via netlink extack")
Signed-off-by: Landen Chao <landen.chao@mediatek.com>
Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mediatek/mtk_eth_soc.c

@@ -2891,6 +2891,8 @@ static int mtk_add_mac(struct mtk_eth *eth, struct device_node *np)
 	eth->netdev[id]->irq = eth->irq[0];
 	eth->netdev[id]->dev.of_node = np;
 
+	eth->netdev[id]->max_mtu = MTK_MAX_RX_LENGTH - MTK_RX_ETH_HLEN;
+
 	return 0;
 
 free_netdev: