libfdt: fix error code of fdt_count_strings()

Currently, this function returns a positive value on error,
so we never know whether this function has succeeded or failed.

For example, if the given property is not found, fdt_getprop()
returns -FDT_ERR_NOTFOUND, and then this function inverts it,
i.e., returns FDT_ERR_NOTFOUND (=1).

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Fixes: bc4147ab ("fdt: Add a function to count strings")
Acked-by: Simon Glass <sjg@chromium.org>