qeth: tagging with VLAN-ID 0

This patch adapts qeth to handle tagged frames with VLAN-ID 0 and
with or without priority information in the tag. It enables qeth to
receive priority-tagged frames on a base interface, for example from
z/OS, without configuring an additional VLAN interface.

Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>