staging: comedi: gsc_hpdi: remove 'volatile' from the private data members

As reported by checkpatch.pl, the private data members do not need
the volatile tag. 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>