Staging: iio: Staticise non-exported functions

Staticise a number of functions which aren't exported outside their
source files, and also add a __user annotation for the buffer in
iio_event_chrdev_read().

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>