staging: comedi: dt2811: tidy up Digital Input/Output register defines

The digital input and outputs are separate ports even though they share the
same register offset. For aesthetics, define then separately and remove the
redundant information in the comment.

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>