Fix wire protocol mismatch on hello listen port

The port number of connected peers appears to be invalid when calling http://127.0.0.1:30203/api/v0alpha/peers/connected

Example response:
```
{
  peers: [
    {
      ip: "54.72.69.180",
      node_id: "bad7236033711b88406656b5947c2f2c4d0e28d5445440982608934645d0ef704f1cf2a62847a5ed1af21b99bbd07eded305848fc00409a81276defaeaba46e5",
      port: 1
    }
  ]
}
```

Expected would be port 30303 instead of 1.

This exposes a protocol mismatch on processing the `hello` peer listen port. `pyethereum` was reading the `CAPABILITIES` field instead of `LISTEN_PORT`.