staging: comedi: ni_mio_common: remove forward declaration 12

Remove the need for the forward declaration of ni_cdio_cancel() by
indirectly calling it using the subdevice (*cancel).

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>