staging: comedi: usbduxsigma: tidy up analog input 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, send the command to start
the analog input sampling, initialize the counter for the conversion,
then either read the acquisition data (submit the urbs) or set the
trigger to read the data later.

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>