staging: comedi: adl_pci9118: clarify the digital I/O register

The same register is used for both the digital inputs and outputs.
Reading the register returns the state of the 4 digtial input channels
and the 4 digital output channels. Writing to the register updates the
digital outputs.

Add some comments to clarify this and use a single define for the register.

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>