net: ftmac100: set max_mtu to allow DSA overhead setting

In case ftmac100 is used with a DSA switch, Linux wants to set MTU
to 1504 to accommodate for DSA overhead. With the default max_mtu
it leads to the error message:
 ftmac100 92000000.mac eth0: error -22 setting MTU to 1504 to include DSA overhead

ftmac100 supports packet length 1518 (MAX_PKT_SIZE constant), so it is
safe to report it in max_mtu.

Signed-off-by: Sergei Antonov <saproj@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20220821160844.474277-1-saproj@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/faraday/ftmac100.c

@@ -1075,6 +1075,7 @@ static int ftmac100_probe(struct platform_device *pdev)
 	SET_NETDEV_DEV(netdev, &pdev->dev);
 	netdev->ethtool_ops = &ftmac100_ethtool_ops;
 	netdev->netdev_ops = &ftmac100_netdev_ops;
+	netdev->max_mtu = MAX_PKT_SIZE;
 
 	platform_set_drvdata(pdev, netdev);