iio: hid-sensors: Prevent crash during hot-unplug

When hid sensor hub is unplugged, there is a crash in
iio_device_unregister_trigger_consumer.
In a typical IIO driver when remove is called, it will unregister and free
trigger and then it will call iio_device_free.
The function iio_trigger_free() will free the allocated memory for trigger.
If this trigger was assigned to iio_dev->trig, then it should be set to NULL.
Othewise when iio_device_free() is called later, it finally calls
iio_device_unregsister_trigger(), which checks for
       if (indio_dev->trig)
                iio_trigger_put(indio_dev->trig);
If indio_dev->trig is not set to NULL, it calls iio_trigger_put on a bad
pointer causing crash.
This scenerio can happen in any driver, which is storing trigger pointer in
iio_dev structure and following current procedure during remove.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>