dma-mapping: fix the kerneldoc for dma_map_sg_attrs

Add the missing description for the nents parameter, and fix a trivial
misalignment.

Fixes: fffe3cc8 ("dma-mapping: allow map_sg() ops to return negative error codes")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Christoph Hellwig <hch@lst.de>

kernel/dma/mapping.c

@@ -207,6 +207,7 @@ static int __dma_map_sg_attrs(struct device *dev, struct scatterlist *sg,
  * dma_map_sg_attrs - Map the given buffer for DMA
  * @dev:	The device for which to perform the DMA operation
  * @sg:		The sg_table object describing the buffer
+ * @nents:	Number of entries to map
  * @dir:	DMA direction
  * @attrs:	Optional DMA attributes for the map operation
  *