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>