ksz884x: fix mtu for VLAN

The Ethernet header does not account for the addition of a VLAN header.
Full size Ethernet frames containing VLAN header are not processed
because the frame is larger than the resulting hw mtu.

Signed-off-by: Doug Kehn <rdkehn@yahoo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>