[media] uvcvideo: Lock stream mutex when accessing format-related information

The stream mutex protects access to the struct uvc_streaming ctrl,
cur_format and cur_frame fields as well as to the hardware probe
control. Lock it appropriately.

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