staging: comedi: das08: rename and split DAS08JR_DIO

The `DAS08JR_DIO` macro contains the offset to the read-only digital
input register and write-only digital output register on the "JR"
boards.  Replace the macro with two new macros (with the same numeric
value) named `DAS08JR_DI_REG` for the digital input register and
`DAS08JR_DO_REG` for the digital output register, and add some comments.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>