media: vimc: streamer: fix memory leak in vimc subdevs if kthread_run fails

In case kthread_run fails, the vimc subdevices
should be notified that streaming stopped so they can
release the memory for the streaming. Also, kthread should be
set to NULL.

Signed-off-by: Dafna Hirschfeld <dafna.hirschfeld@collabora.com>
Acked-by: Helen Koike <helen.koike@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>