On the switch on the MT7988 SoC, as shown in Block Diagram 8.1.1.3 on page
125 of "MT7988A Wi-Fi 7 Generation Router Platform: Datasheet (Open
Version) v0.1", there are only 4 PHYs. That's port 0 to 3. Set the case for
ports which connect to switch PHYs to '0 ... 3'.