staging: comedi: das1800: clarify the analog input cmd triggers

Clarify the documentation in the comedi driver comment block to
better explain the cmd triggert.

Add a comment to step 2b of the (*do_cmdtest) to clarify the
trigger check.

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