wifi: mt76: mt7996: clear stale link state on full reset

In the Linux kernel, the following vulnerability has been resolved:

wifi: mt76: mt7996: clear stale link state on full reset

After a full chip reset, mac80211 reconfig replays interface, link and
channel context setup. mt7996_vif_link_add() short-circuits when the
link_id is still marked in mvif->valid_links, a state introduced for
postponing link teardown to interface removal. The reset path frees the
link structures without clearing those bits, so the replayed setup never
re-creates dev_info/bss_info/STA records in the restarted firmware and
never re-registers the link wcid, leaving the device inoperative.

The reset path also leaks every allocated MLD index: per-link indices
and the per-vif group/remap indices are re-allocated from scratch during
reconfig, but the old bits stay set in the masks, so repeated full
resets exhaust the index space.

Clear valid_links in the reset vif iterator and reset the MLD index
masks alongside the existing omac_mask clearing.