powerpc/xive: Fix refcount leak in xive_get_max_prio

In the Linux kernel, the following vulnerability has been resolved: powerpc/xive: Fix refcount leak in xive_get_max_prio of_find_node_by_path() returns a node pointer with refcount incremented, we should use of_node_put() on it when done. Add missing of_node_put() to avoid refcount leak.