iio: dummy: events: Add missing break

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: 3e34e650db197 ("iio: dummy: Demonstrate the usage of new channel types")
Signed-off-by: Lars-Peter Clausen <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/dummy/iio_simple_dummy_events.c

···
 				st->event_en = state;
 			else
 				return -EINVAL;
+			break;
 		default:
 			return -EINVAL;
 		}