s390/scm_block: make the number of reqs per HW req configurable

Introduce a module parameter to specify the number of requests
we try to handle with one HW request.

Suggested-by: Peter Oberparleiter <peter.oberparleiter@de.ibm.com>
Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>