media: uvcvideo: Fix XU id print in forward scan

An error message in the forward scan code incorrectly prints the ID of
the source entity instead of the XU entity being scanned. Fix it.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>