Staging: comedi: add rtd520 driver

This adds the rtd520 comedi driver to the build.

From: Dan Christian <dac@ptolemy.arc.nasa.gov>
Cc: David Schleef <ds@schleef.org>
Cc: Frank Mori Hess <fmhess@users.sourceforge.net>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>