dma-mapping: return error code from dma_dummy_map_sg()

The .map_sg() op now expects an error code instead of zero on failure.

The only errno to return is -EINVAL in the case when DMA is not
supported.
Signed-off-by: Martin Oliveira <martin.oliveira@eideticom.com>
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

kernel/dma/dummy.c

@@ -22,7 +22,7 @@ static int dma_dummy_map_sg(struct device *dev, struct scatterlist *sgl,
 		int nelems, enum dma_data_direction dir,
 		unsigned long attrs)
 {
-	return 0;
+	return -EINVAL;
 }
 
 static int dma_dummy_supported(struct device *hwdev, u64 mask)