media: video-mux: Skip dangling endpoints

i.MX6 device tree include files contain dangling endpoints for the
board device tree writers' convenience. These are still included in
many existing device trees.
Treat dangling endpoints as non-existent to support them.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Fixes: 612b385e ("media: video-mux: Create media links in bound notifier")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>