swiotlb-xen: fix error code returned by xen_swiotlb_map_sg_attrs

map_sg returns the number of elements mapped, not a dma_addr_t.
In case of error return 0, not DMA_ERROR_CODE.
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>

drivers/xen/swiotlb-xen.c

@@ -548,7 +548,7 @@ xen_swiotlb_map_sg_attrs(struct device *hwdev, struct scatterlist *sgl,
 				xen_swiotlb_unmap_sg_attrs(hwdev, sgl, i, dir,
 							   attrs);
 				sg_dma_len(sgl) = 0;
-				return DMA_ERROR_CODE;
+				return 0;
 			}
 			sg->dma_address = xen_phys_to_bus(map);
 		} else {