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>