staging: comedi: adl_pc19111: remove 'stop_is_none' from private data

This member of the private data can be determined by checking the
cmd->stop_src. Do that instead.

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>