media: cec-core.c: stop kthread_config before kthread

The kthread_config relies on the main kthread (message processing
loop) to be present, so stop kthread_config before kthread.

It's unlikely to be a problem (and I've never seen any issues), but
if nothing else it makes sense to stop the threads in this order.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>