staging: comedi: rtd520: add a few comments

H Hartley Sweeten's recent series of patches to clean up the rtd520
driver made some of the register accesses harder to understand.  Add a
few comments to provide some clues to the reader.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>