leds: powernv: add of_node_put()

of_find_node_by_path() acquires a reference to the node returned by
it and that reference needs to be dropped by its caller.
powernv_led_probe() doesn't do that, so fix it.

Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>