wifi: mt76: mt7996: don't leak MLD group index on remap alloc failure

mt7996_change_vif_links() sets the mld_idx_mask group bit before
allocating the remap index. If the remap allocation fails it jumped to
the exit without clearing that bit, permanently consuming one of the 16
MLD group slots. Release the group bit on the error path.

Fixes: 4fb3b4e7d1 ("wifi: mt76: mt7996: fix MLD group index assignment")
Link: https://patch.msgid.link/20260724124813.3961474-9-nbd@nbd.name
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/mt7996/main.c

@@ -2458,6 +2458,7 @@ mt7996_change_vif_links(struct ieee80211_hw *hw, struct ieee80211_vif *vif,
 
 		idx = get_free_idx(dev->mld_remap_idx_mask, 0, 15) - 1;
 		if (idx < 0) {
+			dev->mld_idx_mask &= ~BIT_ULL(mvif->mld_group_idx);
 			ret = -ENOSPC;
 			goto out;
 		}