staging: comedi: ni_mio_common: adds finite regeneration to AO output

This patch implements for analog output the reinterpretation of stop_arg
when stop_src == TRIG_NONE to allow the user to specify the length of the
buffer that should be repeated.  The intent is to 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.

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>