iio: dummy: events: Add missing break

commit be94a6f6 upstream.

Add missing break in iio_simple_dummy_write_event_config() for the voltage
threshold event enable attribute. Without this writing to the
in_voltage0_thresh_rising_en always returns -EINVAL even though the change
was correctly applied.

Fixes: 3e34e650 ("iio: dummy: Demonstrate the usage of new channel types")
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>