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

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

[ Upstream commit 479ffee68d59c599f8aed8fa2dcc8e13e7bd13c3 ]

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.

Signed-off-by: Bert Karwatzki <spasswolf@web.de>
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://patch.msgid.link/20240812104542.80760-1-spasswolf@web.de
Signed-off-by: Sasha Levin <sashal@kernel.org>