staging: comedi: usbdux: tidy up usbdux_pwm_write()

Remove the sanity check of the private data. This function can only be
called if the private data was successfully allocated in the attach.

Tidy up the function a bit.

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>