of: overlay: Fix forgotten reference to of_overlay_apply()

While technically the ovcs_id is still returned by of_overlay_apply(),
this is an internal function.  All public callers of of_overlay_remove()
pass an ovcs_id returned by the public function of_overlay_fdt_apply().

Fixes: 39a751a4 ("of: change overlay apply input data from unflattened to FDT")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rob Herring <robh@kernel.org>