[media] uvcvideo: uvc_scan_fallback() for webcams with broken chain

[media] uvcvideo: uvc_scan_fallback() for webcams with broken chain

Some devices have invalid baSourceID references, causing uvc_scan_chain()
to fail, but if we just take the entities we can find and put them
together in the most sensible chain we can think of, turns out they do
work anyway. Note: This heuristic assumes there is a single chain.

At the time of writing, devices known to have such a broken chain are
  - Acer Integrated Camera (5986:055a)
  - Realtek rtl157a7 (0bda:57a7)

Signed-off-by: Henrik Ingo <henrik.ingo@avoinelama.fi>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>