wifi: mt76: mt7925: switch link STA allocation to RCU lifetime

wifi: mt76: mt7925: switch link STA allocation to RCU lifetime

Allocate mt792x_link_sta with kzalloc() and free it with kfree_rcu()
instead of devm-managed memory.

msta->link[] is published via RCU, so the link STA must remain valid
until readers have quiesced after teardown. Manage the object lifetime
with kfree_rcu() to match its RCU-visible publication.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Link: https://patch.msgid.link/20260306232238.2039675-18-sean.wang@kernel.org
Signed-off-by: Felix Fietkau <nbd@nbd.name>