staging: comedi: usbdux: tidy up usbdux_pwm_stop()

For aesthetic reasons, pass the comedi_device pointer to this function
instead of the private data pointer. Rename the local variable used
for the private data pointer to the comedi "norm".

Remove the unnecessary sanity check of the private data pointer. This
function can only be called is the private data was allocated during
the attach.

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