staging: comedi: adv_pci1710: remove 'neverending_ai' from private data

This member of the private data is not necessary. We can determine if
the analog input command is neverending by checking the cmd->stop_src:

  TRIG_COUNT -> !neverending_ai
  TRIG_NONE -> neverending_ai

Do that instead and remove the unnecessary member.

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>