wifi: mt76: mt7996: use hweight16 to get correct tx antenna

The chainmask is u16 so using hweight8 cannot get correct tx_ant.
Without this patch, the tx_ant of band 2 would be -1 and lead to the
following issue:
BUG: KASAN: stack-out-of-bounds in mt7996_mcu_add_sta+0x12e0/0x16e0 [mt7996e]

Fixes: 98686cd ("wifi: mt76: mt7996: add driver for MediaTek Wi-Fi 7 (802.11be) devices")
Signed-off-by: Peter Chiu <chui-hao.chiu@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Link: https://patch.msgid.link/20240816094635.2391-1-shayne.chen@mediatek.com
Signed-off-by: Felix Fietkau <nbd@nbd.name>

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

@@ -1653,7 +1653,7 @@ mt7996_mcu_sta_bfer_tlv(struct mt7996_dev *dev, struct sk_buff *skb,
 {
 	struct mt7996_vif *mvif = (struct mt7996_vif *)vif->drv_priv;
 	struct mt7996_phy *phy = mvif->phy;
-	int tx_ant = hweight8(phy->mt76->chainmask) - 1;
+	int tx_ant = hweight16(phy->mt76->chainmask) - 1;
 	struct sta_rec_bf *bf;
 	struct tlv *tlv;
 	static const u8 matrix[4][4] = {