of: unittest: Fix of_count_phandle_with_args() expected value message

The expected result value for the call to of_count_phandle_with_args()
was updated from 7 to 8, but the accompanying error message was
forgotten.

Fixes: 4dde8356

 ("of: Fix double free in of_parse_phandle_with_args_map")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20240111085025.2073894-1-geert+renesas@glider.be
Signed-off-by: Rob Herring <robh@kernel.org>