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>

Documentation/rapidio/mport_cdev.txt

@@ -80,6 +80,10 @@ functionality of their platform when planning to use this driver:
 
 III. Module parameters
 
+- 'dma_timeout' - DMA transfer completion timeout (in msec, default value 3000).
+        This parameter set a maximum completion wait time for SYNC mode DMA
+        transfer requests and for RIO_WAIT_FOR_ASYNC ioctl requests.
+
 - 'dbg_level' - This parameter allows to control amount of debug information
         generated by this device driver. This parameter is formed by set of
         bit masks that correspond to the specific functional blocks.