staging: comedi: ni_mio_common: Cleans up/clarifies ni_ao_cmd

This patch implements ni_ao_cmd much more closely organized like NI MHDDK
examples and DAQ-STC pseudo-code.  Adds comments with some more specific
references to the DAQ-STC.

For stop_src==TRIG_NONE (continuous output mode of entire buffer), the
count for the UC counter was corrected to represent the maximum count
possible (0xffffff).  Prior behavior for stop_src=TRIG_NONE did not
actually follow the DAQ-STC.  Furthermore, stop_src==TRIG_NONE now
correctly uses code specialized for either m-series or e-series devices.

It should be noted that stop_src==TRIG_NONE does _not_ with this patch
(or with prior behavior in ni_mio_common) actually implement true
continuous output.  Rather, the output is simply configured to operate as a
single buffer output, but where the buffer is as large as is possible with
NI-STC hardware.
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>