libfdt: fix comment block of fdt_get_property_namelen()

The statement "Identical to fdt_get_property_namelen() ..." does not
make sense for the comment of fdt_get_property_namelen() itself.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>