s390-ccw.img: Consume service interrupts

We have to consume the outstanding service interrupt after each
service call, otherwise a correct implementation will return
CC=2 on subsequent service calls.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>