use sg helper function in DMA mapping documentation

Signed-off-by: Saeed Bishara <saeed.bishara@gmail.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

Documentation/DMA-mapping.txt

@@ -514,7 +514,7 @@ With scatterlists, you map a region gathered from several regions by:
 	int i, count = pci_map_sg(dev, sglist, nents, direction);
 	struct scatterlist *sg;
 
-	for (i = 0, sg = sglist; i < count; i++, sg++) {
+	for_each_sg(sglist, sg, count, i) {
 		hw_address[i] = sg_dma_address(sg);
 		hw_len[i] = sg_dma_len(sg);
 	}