staging: comedi: c6xdigio: define the data register bits

To clarify the code, define the bits in the data register and remove the
magic numbers.

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>