s390/qeth: merge TX skb mapping code

Merge the __qeth_fill_buffer() helper into its only caller. This way all
mapping-related context is in one place, and we can make some more use
of it in a subsequent patch.

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