s390/qeth: pass correct length to header_ops->create()

We need to pass the *payload* length, not the L2 address length.
For qeth (using eth_header()) this is merely a cosmetic change:
the parameter only matters when building headers for ETH_P_802_2
or ETH_P_802_3, whereas our fake headers are built with
ETH_P_IP / ETH_P_IPV6 / 0.

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