staging: comedi: usbduxfast: don't pass the private data directly

Instead of passing the private data to the internal functions, pass
the comedi_device pointer and get the private data from it.

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>