staging: iio: industrialio-trigger: set iio_poll_func private_data

Failure to set iio_poll_func private_data, causes zero pointer access
violations in all consumer trigger handlers.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>