of: overlay: unittest: overlay_bad_unresolved: Spelling s/ok/okay/

While "ok" is recognized, the proper status value for an operational
device is "okay".

Fixes: eb38b952

 ("of: overlay: unittest: Add test for unresolved symbol")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/923f4f605b86f23d001c6efc9c2237ab449d447d.1698228277.git.geert+renesas@glider.be
Signed-off-by: Rob Herring <robh@kernel.org>