s390: qeth: wrong packet length in qdio header

Packets Length in qdio header is broken when using
EDDP on Layer2 devices. This leads to skb_under_panic on receiver
system when running on z/VM GuestLAN devices.

Signed-off-by: Frank Pavlic <fpavlic@de.ibm.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>