staging: comedi: usbduxsigma: cleanup the stop functions

The 'this_usbduxsub' parameter is actually the comedi_device private data,
rename it to 'devpriv' which is more common in comedi drivers.

Remove the unnecessary pr_err() noise in usbdux_ai_stop().

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>