iio: mma8452: Fix trigger reference couting

commit cd008223 upstream.

The mma8452 driver directly assigns a trigger to the struct iio_dev. The
IIO core when done using this trigger will call `iio_trigger_put()` to drop
the reference count by 1.

Without the matching `iio_trigger_get()` in the driver the reference count
can reach 0 too early, the trigger gets freed while still in use and a
use-after-free occurs.

Fix this by getting a reference to the trigger before assigning it to the
IIO device.

Fixes: ae6d9ce0

 ("iio: mma8452: Add support for interrupt driven triggers.")
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/20211024092700.6844-1-lars@metafoo.de
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>