s390/scm_blk: allow more cluster size values

Allow 0 and powers of 2 between 2 and 128 for write_cluster_size.

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