[media] stk-webcam: add support for control events and prio handling

Also correct the first_init static: this should be part of the stk_camera struct.

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>