iio: adc: ad_sigma_delta: 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>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Cc: Stable@vger.kernel.org

drivers/iio/adc/ad_sigma_delta.c

@@ -472,7 +472,7 @@ static int ad_sd_probe_trigger(struct iio_dev *indio_dev)
 		goto error_free_irq;
 
 	/* select default trigger */
-	indio_dev->trig = sigma_delta->trig;
+	indio_dev->trig = iio_trigger_get(sigma_delta->trig);
 
 	return 0;