net: dsa: microchip: enable MTU normalization for KSZ8795 and KSZ9477 compatible switches

KSZ8795 and KSZ9477 compatible series of switches use global max frame
size configuration register. So, enable MTU normalization for this reason.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Acked-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>