net: ethernet: Properly handle VLAN tag 0

Packets are forwarded to the native interface or in other words,
the vlan header is simply stripped and ignored. This feature is called
'priority tagging'.

Signed-off-by: Cla Mattia Galliard <cla-mattia.galliard@zuehlke.com>
Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>