staging: comedi: usbdux: tidy up usbdux_pwm_pattern()

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

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 to make it more concise.

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>