staging: comedi: s526: add defines for the ao/ai data register

The same register is used for analog output and input data. Add defines
for both for added clarity.

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>