[media] uvcvideo: Send control change events for slave ctrls when the master changes

This allows v4l2 control UI-s to update the inactive state (ie grey-ing
out of controls) for slave controls when the master control changes.

[Use __uvc_find_control() to find slave controls, as they're always
located in the same entity as the corresponding master control]

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>