pc-bios/s390-ccw: provide entry selection on LOADPARM for SCSI disk

Fix SCSI bootmap interpreter to make use of any specified entry of the
Program Table using the leftmost numeric value from the LOADPARM, if specified.

Initial patch from Eugene (jno) Dvurechenski.

Signed-off-by: Eugene (jno) Dvurechenski <jno@linux.vnet.ibm.com>
Signed-off-by: Farhan Ali <alifm@linux.vnet.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>