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>