staging: comedi: s626: rename ai_continous

Rename the `ai_continous` member of `struct s626_private` to
`ai_continuous`.

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