s390/qeth: clean up fill_buffer() offset logic

For some xmit paths we pass down a data offset to qeth_fill_buffer(),
to indicate that the first k bytes of the skb should be skipped when
mapping it into buffer elements.
Commit acd9776b ("s390/qeth: no ETH header for outbound AF_IUCV")
recently switched the offset for the IUCV-over-HiperSockets path
from 0 to ETH_HLEN, and now we have

	device	offset
	OSA	= 0
	IQD	> 0

for all xmit paths.

OSA would previously pass down -1 from do_send_packet(), to distinguish
between 1) OSA and 2) IQD with offset 0. That's no longer needed now,
so have it pass 0, make the offset unsigned and clean up how we apply
the offset in __qeth_fill_buffer().

No change of behaviour for any of our current xmit paths.

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>