staging: comedi: amplc_dio200: rename enum dio200_layout

Rename `enum dio200_layout` to `enum dio200_layout_idx`.  It's only used
once and frees up the `dio200_layout` tag for something else later.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>