staging: comedi: ni_at_ao: tidy up digital i/o register defines

The digital in and digital out registers on this board share the same
address. They also effect the same digital i/o pins. To make this clear
combine the two defines.

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>