qeth: Prepare-function to call s390dbf was wrong

Prepare-function to call s390dbf was wrong handling variable arguments.
This worked as macro but not as function any more.
Now using va_list processing.

Signed-off-by: Peter Tiedemann <ptiedem@de.ibm.com>
Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>