staging: comedi: pcl818: remove 'neverending_ai' from private data

This member of the private data is not needed. We can check the stop_src
to determine this information:

  neverending_ai -> cmd->stop_src == TRIG_NONE
  !neverending_ai -> cmd->stop_src == TRIG_COUNT

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>