qeth module size reduction.

Replace complex macro for s390dbf calls by equivalent function. This reduces
module size about 10% without visible performance impact.

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>