staging: iio: remove now defunct header definitions and add some statics

Now we have combined all chrdevs into one, some bits and bobs are only
needed in industrialio-core.c and don't need to be in the core header.

Also remove some unused defines.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>