staging: comedi: usbduxsigma: rename usbdux_pwm_stop()

Rename the function so it has namespace associated with the driver.

For aesthetic reasons, pass the comedi_device pointer instead of the
private data pointer as the first parameter.

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>