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>