CVE-2021-47500 iio: mma8452: Fix trigger reference couting

In the Linux kernel, the following vulnerability has been resolved:

iio: mma8452: Fix trigger reference couting

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.