media: vivid: fix an error code in vivid_create_instance()

Preserve the error code stored in "dev->kthread_cec" before setting it
to NULL.

Fixes: 439e520995ab ("media: vivid: add signal-free time for cec message xfer")
Signed-off-by: Dan Carpenter <[e-mail]>
Signed-off-by: Hans Verkuil <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

drivers/media/test-drivers/vivid/vivid-core.c

···
 		dev->kthread_cec = kthread_run(vivid_cec_bus_thread, dev,
 					       "vivid_cec-%s", dev->v4l2_dev.name);
 		if (IS_ERR(dev->kthread_cec)) {
+			ret = PTR_ERR(dev->kthread_cec);
 			dev->kthread_cec = NULL;
 			v4l2_err(&dev->v4l2_dev, "kernel_thread() failed\n");
-			ret = PTR_ERR(dev->kthread_cec);
 			goto unreg_dev;
 		}
 	}