s390/pci: cleanup clp inline assembly

Tell gcc that the memory region pointed to by req will be used (and
changed). Also remove the (now) superfluous memory constraint.

Acked-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>