mediatek: mt76: eeprom: fix missing of_node_put() in mt76_find_power_limits_node()

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

Fixes: 22b980ba

 ("mt76: add functions for parsing rate power limits from DT")
Signed-off-by: Liang He <windhl@126.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>