staging: comedi: das16m1: tidy up digital input/output register defines

The digtial inputs and outputs are read/written using the same register
offset but they are different logical registers. Physically they are the
same register with the hi 4 bits returning the inputs and the lo 4 bits
driving the outputs.

For aesthetics, use two different defines for the registers.

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>