[media] v4l: vsp1: Fix routing cleanup when stopping the stream

Commit d9b45ed3 ("v4l: vsp1: Support
multi-input entities") reworked pipeline routing configuration and
introduced a bug by writing to the entities routing registers without
first checking whether the entity had a routing register. This results
in overwriting the value at offset 0 of the device register space when
stopping the stream.

Fix this by skipping routing register write for entities without a
routing register.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>