scsi: cxlflash: Migrate scsi command pointer to AFU command

Currently, when sending a SCSI command, the pointer is stored in a
reserved field of the AFU command descriptor for retrieval once the
SCSI command has completed. In order to support new descriptor formats
that make use of the reserved field, the pointer is migrated to outside
the descriptor where it can still be found during completion processing.

Signed-off-by: Matthew R. Ochs <mrochs@linux.vnet.ibm.com>
Acked-by: Uma Krishnan <ukrishn@linux.vnet.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>