iio: adc: ad_sigma_delta: Fix indio_dev->trig assignment

iio: adc: ad_sigma_delta: Fix indio_dev->trig assignment

commit 9e5846be33277802c0c76e5c12825d0e4d27f639 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>