staging: comedi: amplc_dio200_common: remove 'continuous' from subdevice private data

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

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