staging: comedi: c6xdigio: define the register map

The register map was already defined but it was not being used. Rename
to defines and use them in the code.

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>