staging: comedi: usbduxfast: remove 'comedidev' from the private data

Now that the comedi_device is passed to the internal functions, the
back pointer to it is not needed in the private data. Remove 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>