s390/qeth: fix up protocol headers early

When qeth_add_hw_header() falls back to the HW header cache, it also
copies over the necessary protocol headers. Thus any manipulation to
the protocol headers needs to happen before adding the HW header.

For current usage this doesn't matter, but it becomes relevant when
moving TSO transmission over to the faster code path.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>