libfdt: fix error code of fdt_get_string_index()

As mentioned in the comment block in include/libfdt.h,
fdt_get_string_index() is supposed to return a negative value
on error.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Fixes: 5094eb40 ("fdt: Add functions to retrieve strings")
Acked-by: Simon Glass <sjg@chromium.org>