lpfc: Fix scsi prep dma buf error.

Didn't check for less-than-or-equal zero. Means we may later call
scsi_dma_unmap() even though we don't have valid mappings.

Signed-off-by: Dick Kennedy <dick.kennedy@avagotech.com>
Signed-off-by: James Smart <james.smart@avagotech.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: James Bottomley <JBottomley@Odin.com>

drivers/scsi/lpfc/lpfc_scsi.c

@@ -3257,7 +3257,7 @@ lpfc_scsi_prep_dma_buf_s4(struct lpfc_hba *phba, struct lpfc_scsi_buf *lpfc_cmd)
 		 */
 
 		nseg = scsi_dma_map(scsi_cmnd);
-		if (unlikely(!nseg))
+		if (unlikely(nseg <= 0))
 			return 1;
 		sgl += 1;
 		/* clear the last flag in the fcp_rsp map entry */