wifi: mt76: fix coverity overrun-call in mt76_get_txpower()

Make sure the nss is valid for nss_delta array. Return zero
if the index is invalid.

Coverity message:
Event overrun-call: Overrunning callee's array of size 4 by passing
argument "n_chains" (which evaluates to 15) in call to
"mt76_tx_power_nss_delta".
int delta = mt76_tx_power_nss_delta(n_chains);

Fixes: 07cda406 ("mt76: fix rounding issues on converting per-chain and combined txpower")
Signed-off-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/mt76.h

@@ -1110,8 +1110,9 @@ static inline bool mt76_is_skb_pktid(u8 pktid)
 static inline u8 mt76_tx_power_nss_delta(u8 nss)
 {
 	static const u8 nss_delta[4] = { 0, 6, 9, 12 };
+	u8 idx = nss - 1;
 
-	return nss_delta[nss - 1];
+	return (idx < ARRAY_SIZE(nss_delta)) ? nss_delta[idx] : 0;
 }
 
 static inline bool mt76_testmode_enabled(struct mt76_phy *phy)