wifi: mt76: mt7915: add missing of_node_put()

Add missing of_node_put() after of_reserved_mem_lookup()

Fixes: 99ad32a4 ("mt76: mt7915: add support for MT7986")
Signed-off-by: Wang Yufen <wangyufen@huawei.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>