wifi: mt76: mt7915: avoid nss underflow in mt7915_mcu_get_sta_nss

If a peer's VHT/HE MCS map has no supported spatial stream (all fields
0x3), the loop exits with nss == 0 and the function returned (u8)-1 (255),
which was then written into the firmware sta_rec_bf beamforming fields.
Clamp the result to 0.

Fixes: 89029a8548 ("mt76: mt7915: add Tx beamformer support")
Link: https://patch.msgid.link/20260722082610.2699628-9-nbd@nbd.name
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/mt7915/mcu.c

@@ -51,7 +51,7 @@ mt7915_mcu_get_sta_nss(u16 mcs_map)
 			break;
 	}
 
-	return nss - 1;
+	return nss ? nss - 1 : 0;
 }
 
 static void