staging: comedi: pcmuio: remove unused private data variables

These members of the private data are either not referenced or are set
but never used by the driver. Remove them.

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>