staging: comedi: amplc_pci230: remove 'ao_continuous' 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>