net/dsa/hirschmann: Add missing of_node_get() in hellcreek_led_setup()

of_find_node_by_name() will decrease the refcount of its first arg and
we need a of_node_get() to keep refcount balance.

Fixes: 7d9ee2e8

 ("net: dsa: hellcreek: Add PTP status LEDs")
Signed-off-by: Liang He <windhl@126.com>
Link: https://lore.kernel.org/r/20220622040621.4094304-1-windhl@126.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>