staging: comedi: das08: rename DAS08_TRIG_12BIT

The `DAS08_TRIG_12BIT` macro contains the offset to the write-only
software trigger register for 12-bit or 16-bit analog-to-digital
conversions.  Rename the macro to `DAS08_AI_TRIG_REG` and add a comment.

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>