staging: comedi: usbdux: tidy up usbdux_free_usb_buffers()

For aesthetics, pass the comedi_device pointer to this function
instead of the private data pointer.

The usb_kill_urb() calls are not needed. All the urbs were unlinked
before this function was called.

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>