net: lan966x: Fix the MTU calculation

When the MTU was changed, the lan966x didn't take in consideration
the L2 header and the FCS. So the HW was configured with a smaller
value than what was desired. Therefore the correct value to configure
the HW would be new_mtu + ETH_HLEN + ETH_FCS_LEN.
The vlan tag is not considered here, because at the time when the
blamed commit was added, there was no vlan filtering support. The
vlan fix will be part of the next patch.

Fixes: d28d6d2e ("net: lan966x: add port module support")
Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>