mediatek: mt76: mac80211: Fix missing of_node_put() in mt76_led_init()

We should use of_node_put() for the reference 'np' returned by
of_get_child_by_name() which will increase the refcount.

Fixes: 17f1de56 ("mt76: add common code shared between multiple chipsets")
Signed-off-by: Liang He <windhl@126.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>