staging: comedi: ni_tio.h: remove subdev_to_counter()

This inline helper function simply returns the s->private void *.
Remove the helper and just get the s->private void * directly where
needed.

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>