wifi: mt76: mt7996: fix out-of-bounds array access during hardware restart

wifi: mt76: mt7996: fix out-of-bounds array access during hardware restart

During hardware restart, link_id can be IEEE80211_LINK_UNSPECIFIED,
causing an out-of-bounds array access on msta->link[].

Add mt7996_sta_link() and mt7996_sta_link_protected() helper functions
for accessing sta links with proper RCU handling and bounds checking.
Use them for any sta link RCU access.

Reported-by: Chad Monroe <chad@monroe.io>
Link: https://patch.msgid.link/20260324154904.2555603-1-nbd@nbd.name
Signed-off-by: Felix Fietkau <nbd@nbd.name>