media: dt-bindings: techwell,tw9900: Fix port schema ref

The port@0 node doesn't define any extra properties in the port or endpoint
nodes, so the $ref should be to "/properties/port" instead as it restricts
extra properties.

Fixes: 0f82ffa9 ("media: dt-bindings: media: i2c: Add bindings for TW9900")
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>