of: dynamic: Fix race in getting old property when updating property

__of_update_property() returns the existing property if there is one, but
that value is never added to the changeset. Updates work because the
existing property was also retrieved before in of_changeset_action(),
but that is racy as of_changeset_action() doesn't hold any locks. The
property could be changed before the changeset is applied.

Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20230801-dt-changeset-fixes-v3-4-5f0410e007dd@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>