staging: comedi: usbduxfast: remove unnecessary clearing of private data

The clearing of the private data members in the (*detatch) is not
necessary. The comedi core is going to kfree the private data as soon
as the (*detach) finishes.

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>