rapidio: fix return value description for dma_prep functions

Update return value description for rio_dma_prep_...  functions to
include error-valued pointer that can be returned by HW mport device
drivers.  Return values from these functions must be checked using
IS_ERR_OR_NULL macro.

This patch is applicable to kernel versions starting from v4.6-rc1.

Link: http://lkml.kernel.org/r/1469125134-16523-4-git-send-email-alexandre.bounine@idt.com
Signed-off-by: Alexandre Bounine <alexandre.bounine@idt.com>
Cc: Matt Porter <mporter@kernel.crashing.org>
Cc: Andre van Herk <andre.van.herk@prodrive-technologies.com>
Cc: Barry Wood <barry.wood@idt.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/rapidio/rio.c

@@ -1848,7 +1848,9 @@ EXPORT_SYMBOL_GPL(rio_release_dma);
  * Initializes RapidIO capable DMA channel for the specified data transfer.
  * Uses DMA channel private extension to pass information related to remote
  * target RIO device.
- * Returns pointer to DMA transaction descriptor or NULL if failed.
+ *
+ * Returns: pointer to DMA transaction descriptor if successful,
+ *          error-valued pointer or NULL if failed.
  */
 struct dma_async_tx_descriptor *rio_dma_prep_xfer(struct dma_chan *dchan,
 	u16 destid, struct rio_dma_data *data,
@@ -1883,7 +1885,9 @@ EXPORT_SYMBOL_GPL(rio_dma_prep_xfer);
  * Initializes RapidIO capable DMA channel for the specified data transfer.
  * Uses DMA channel private extension to pass information related to remote
  * target RIO device.
- * Returns pointer to DMA transaction descriptor or NULL if failed.
+ *
+ * Returns: pointer to DMA transaction descriptor if successful,
+ *          error-valued pointer or NULL if failed.
  */
 struct dma_async_tx_descriptor *rio_dma_prep_slave_sg(struct rio_dev *rdev,
 	struct dma_chan *dchan, struct rio_dma_data *data,