qeth: l3 add vlan hdr in passthru frames

OSA l3 mode is hw accelerated VLAN only for IPv4. Take care we
add the vlan hdr to a passthru frame in the device driver.

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