staging: comedi: ni_mio_common: remove forward declaration 18

Move ni_ai_drain_dma() to remove the need for the forward declaration.

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>