s390/qeth: straighten out fill_buffer() interface

1. for adjusting the buffer's next_element_to_fill in __fill_buffer(),
   just pass the full qeth_qdio_out_buffer struct
2. when adding a header element, be consistent about passing
   a hint ('is_first_elem') to __fill_buffer()

No functional change.

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