[media] stk-webcam: enable core-locking

This makes it possible to switch to unlocked_ioctl.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Tested-by: Arvydas Sidorenko <asido4@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>