of: overlay: Make node skipping in init_overlay_changeset() clearer

Make it more clear that nodes without "__overlay__" subnodes are
skipped, by reverting the logic and using continue.
This also reduces indentation level.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rob Herring <robh@kernel.org>