iio: st_sensors: Fix indio_dev->trig assignment

commit f0e84acd7056e6d7ade551c6439531606ae30a46 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>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>