staging: comedi: usbduxsigma: tidy up analog output command support

Rename the (*do_cmdtest) and (*do_cmd) functions so they have namespace
associated with the driver. Rename the local variable used for the
private data pointer.

Move all the command argument initialization and testing into the
(*do_cmdtest) function. That function is always called by the comedi
core before the (*do_cmd) function. The only thing the (*do_cmd) function
should have to do is setup the channel list, initialize the counter for
the conversion, then either start the conversion (submit the urbs) or
set the trigger to start the conversion.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>