net: dsa: microchip: move max mtu to one location

There are no HW specific registers, so we can process all of them
in one location.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Tested-by: Arun Ramadoss <arun.ramadoss@microchip.com> (KSZ9893 and LAN937x)
Signed-off-by: Paolo Abeni <pabeni@redhat.com>