net: eth: Fix ethernet max header size on VLAN presence

Currently NET_ETH_MAX_HDR_SIZE is computed as the size of the
structure struct net_eth_hdr, but this does not take into account
the additional 4 bytes on the ethernet frame header in the presence
of VLAN, fix the issue using the size of struct net_eth_vlan_hdr
when CONFIG_NET_VLAN is enabled.

Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>