staging: comedi: usbdux: tidy up usbdux_pwm_config()

Rename the local variable used for the private data pointer to 'devpriv'.

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>