scsi: fas216: Stop using scsi_cmnd.scsi_done

Store the completion callback pointer in struct fas216_cmd_priv instead of
in struct scsi_cmnd. This patch prepares for removal of the scsi_done
member from struct scsi_cmnd.

Link: https://lore.kernel.org/r/20211007202923.2174984-34-bvanassche@acm.org


Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>