wifi: mt76: mt7921: fix NULL pointer access in mt7921_ipv6_addr_change

In the Linux kernel, the following vulnerability has been resolved: wifi: mt76: mt7921: fix NULL pointer access in mt7921_ipv6_addr_change When disabling wifi mt7921_ipv6_addr_change() is called as a notifier. At this point mvif->phy is already NULL so we cannot use it here.