iio: st_sensors: Fix indio_dev->trig assignment

This can result in wrong reference count for trigger device, call
iio_trigger_get to increment reference.
Refer to http://www.spinics.net/lists/linux-iio/msg13669.html for discussion
with Jonathan.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Cc: Stable@vger.kernel.org

drivers/iio/common/st_sensors/st_sensors_trigger.c

@@ -49,7 +49,7 @@ int st_sensors_allocate_trigger(struct iio_dev *indio_dev,
 		dev_err(&indio_dev->dev, "failed to register iio trigger.\n");
 		goto iio_trigger_register_error;
 	}
-	indio_dev->trig = sdata->trig;
+	indio_dev->trig = iio_trigger_get(sdata->trig);
 
 	return 0;