staging: comedi: comedi_parport: remove 'c_data' from private data

The control register for the parallel port is readable so there is no need
to cache the current value in the private data.

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>