iio: imu: Use iio_trigger_get for indio_dev->trig assignment

This patch uses iio_trigger_get to increment the reference
count of trigger device, to avoid incorrect assignment.
Can result in a null pointer dereference during removal if the
trigger has been changed before removal.

This patch refers to a similar situation encountered through the
following discussion:
http://www.spinics.net/lists/linux-iio/msg13669.html

Signed-off-by: Darshana Padmadas <darshanapadmadas@gmail.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/imu/adis_trigger.c

@@ -60,7 +60,7 @@ int adis_probe_trigger(struct adis *adis, struct iio_dev *indio_dev)
 	iio_trigger_set_drvdata(adis->trig, adis);
 	ret = iio_trigger_register(adis->trig);
 
-	indio_dev->trig = adis->trig;
+	indio_dev->trig = iio_trigger_get(adis->trig);
 	if (ret)
 		goto error_free_irq;