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