staging: comedi: usbduxsigma: change return type of the stop functions

The usbdux_pwm_cancel() function is the only caller that does not validate
the comedi_device private data before calling the stop function. Move the
validation test to that function and remove the unnecessary sanity checks.

Since the stop functions always succeed, change the return type to void.

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>