media: uvcvideo: Support devices that report an OT as an entity source

Some devices reference an output terminal as the source of extension
units. This is incorrect, as output terminals only have an input pin,
and thus can't be connected to any entity in the forward direction. The
resulting topology would cause issues when registering the media
controller graph. To avoid this problem, connect the extension unit to
the source of the output terminal instead.

While at it, and while no device has been reported to be affected by
this issue, also handle forward scans where two output terminals would
be connected together, and skip the terminals found through such an
invalid connection.

Reported-and-tested-by: John Nealy <[email protected]>

Signed-off-by: Laurent Pinchart <[email protected]>
Signed-off-by: Hans de Goede <[email protected]>
Signed-off-by: Mauro Carvalho Chehab <[email protected]>

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