staging: comedi: s626: remove 'ai_continuous' from private data

This member of the private data is set based on the cmd->stop_src. Just use the
cmd->stop_src directly and remove the 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>