s390/qeth: pass full data length to l2_fill_header()

For IQD we already need to fix up the qeth_hdr's length field, and
future changes will require more flexibility for OSA as well. The
device-specific path knows best what header length it requires, so just
pass it from there.
While at it, remove the unused qeth_card parameter.

No functional change.

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