scsi: scsi_transport_spi: Use scsi_cmd_to_rq() instead of scsi_cmnd.request

Prepare for removal of the request pointer by using scsi_cmd_to_rq()
instead. This patch does not change any functionality.

Link: https://lore.kernel.org/r/[e-mail]
Cc: Christoph Hellwig <[e-mail]>
Cc: Hannes Reinecke <[e-mail]>
Cc: Ming Lei <[e-mail]>
Reviewed-by: Hannes Reinecke <[e-mail]>
Signed-off-by: Bart Van Assche <[e-mail]>
Signed-off-by: Martin K. Petersen <[e-mail]>

drivers/scsi/scsi_transport_spi.c

···
 {
         if (cmd->flags & SCMD_TAGGED) {
 		*msg++ = SIMPLE_QUEUE_TAG;
-        	*msg++ = cmd->request->tag;
+		*msg++ = scsi_cmd_to_rq(cmd)->tag;
         	return 2;
 	}