of: overlay: Simplify of_overlay_fdt_apply() tail

It does not hurt to fill in the changeset id while the mutex is still
held.  After doing so, the function tails for the success and failure
cases become identical, so they can be unified.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Frank Rowand <frank.rowand@sony.com>
Tested-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/6a3357a8f7f29704350e3ffae768ee8a462b54d3.1657893306.git.geert+renesas@glider.be