net: lan966x: Adjust maximum frame size when vlan is enabled/disabled

When vlan filtering is enabled/disabled, it is required to adjust the
maximum received frame size that it can received. When vlan filtering is
enabled, it would all to receive extra 4 bytes, that are the vlan tag.
So the maximum frame size would be 1522 with a vlan tag. If vlan
filtering is disabled then the maximum frame size would be 1518
regardless if there is or not a vlan tag.

Fixes: 6d2c186a

 ("net: lan966x: Add vlan support.")
Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>