qeth: Fix IP version detection for VLAN traffic

The current code would always return 0 for VLAN-encapsulated IP traffic.
One notable side effect was that VLAN traffic would never get prioritized
on OSD and OSX devices when priority queueing modes prio_queueing_tos or
prio_queueing_prec were enabled.

Signed-off-by: Stefan Raspl <raspl@linux.vnet.ibm.com>
Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>