staging: comedi: das08: rename DAS08_LSB and DAS08_MSB

The `DAS08_LSB` and `DAS08_MSB` macros contain the offsets to the
least-significant and most-significant analog input data registers.
Rename them to `DAS08_AI_LSB_REG` and `DAS08_AI_MSB_REG` respectively
and add comments to document them.

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>