s390/qeth: unify code to build header elements

After plenty of refactoring, use hd_len as single indication that
the skb needs a dedicated header element.

This preserves existing behaviour for TSO, as 'hdr' always points
to skb->data.

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