staging:iio:events: Make sure userspace buffer is large enough

Make sure that the userspace buffer is large enough to hold a iio_event_data
struct before writing to it.

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