staging:iio: Introduce iio_core.h and move all core only stuff out of iio.h.

Also get rid of a few function defs where they are only now in one core file
anyway.
Whilst here add mask = 0 to get rid of warning.

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