This patch continues the implementation of reinterpreting stop_arg when
stop_src == TRIG_NONE for national instruments cdio output on e/m-series
devices.  This is part of a series of patches that allow a user to have a
specific buffer repeated as-is indefinitely.  The contents of the DMA
buffer can be left static or changed by the user via mmap access to the DMA
buffer.  If the contents are changed by the user, additional munging is not
performed by the driver and only a single call to
comedi_mark_buffer_written should be done.  The original behavior is
preserved when stop_arg == 0, as would be the prior use case.

As opposed to analog output, this patch is relatively simple.  First, the
digital output capabilities are much more limited/simple as compared to the
analog output device on NI e/m-series hardware, and second, this patch
relies on changes made with the earlier patch to accomplish limiting the
DMA buffer transfer.
Signed-off-by: Spencer E. Olson <olsonse@umich.edu>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>