staging: comedi: ni_mio_common: remove forward declaration 20

Move ni_ai_insn_read() to remove the need for a 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>