fix an example of scatterlists handling in DMA-API.txt

This example isn't the proper way to handle scatterlists (can't handle
sg chaining).

Signed-off-by: FUJITA Tomonori <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

Documentation/DMA-API.txt

···
 	int i, count = dma_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);
 	}