staging: comedi: das08: rename DAS08_CONTROL

The `DAS08_CONTROL` macro contains the offset to the write-only control
register.  Rename it to `DAS08_CONTROL_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>