rapidio/documentation/mport_cdev: add missing parameter description

rapidio/documentation/mport_cdev: add missing parameter description

Add missing description for rio_mport_cdev driver parameter
'dma_timeout'.

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

Link: http://lkml.kernel.org/r/20160901173104.2928-1-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>