staging:iio: Update iio_event_interface documentation

The documentation for the iio_event_interface does not match the actual struct
anymore. This patch removes the documentation for non-existing fields and adds
documentation for missing fields.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>