of: unittest: Remove bogus overlay mutex release from overlay_data_add()

overlay_data_add() never takes the special overlay mutex, so it must not
be released in the error patch.

Presumably the call to of_overlay_mutex_unlock() is a relic from v1 of
the patch.

Fixes: f948d6d8 ("of: overlay: avoid race condition between applying multiple overlays")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Rob Herring <robh@kernel.org>