scsi: sym53c500_cs: Move the SCSI pointer to private command data

Set .cmd_size in the SCSI host template instead of using the SCSI pointer
from struct scsi_cmnd.
This patch prepares for removal of the SCSI pointer from struct scsi_cmnd.

Link: https://lore.kernel.org/r/20220218195117.25689-40-bvanassche@acm.org


Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Himanshu Madhani <himanshu.madhani@oracle.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>