Is it possible to use fixed port for listening
This plugin is awesome. However, looks like the listening port is changed every time after reboot. Is that possible to use a fixed port number?  That will help me to set up firewall rule (ufw) for this plugin.

### Expected Behavior
Use a fixed port to receive the device message.

### Current Behavior
the listening port is changed every time after reboot

### Steps to Reproduce (for bugs)
Enable the firewall.
Allow incoming message from specific port for this plugin
Restart the homebridge.
Port is changed. No longer get update of device

### Versions
v10.15.2
0.4.50
Raspbian 10

### Configuration
```json
        {
            "platform": "TplinkSmarthome",
            "name": "TplinkSmarthome",
            "broadcast": "192.168.1.255",
            "devices": [],
            "deviceTypes": [],
            "macAddresses": [],
            "excludeMacAddresses": [],
            "pollingInterval": 10,
            "addCustomCharacteristics": true,
            "inUseThreshold": 0,
            "switchModels": [
                "HS200",
                "HS210"
            ],
            "timeout": 15
        }
```

### Homebridge Log / Command Output
Homebridge:
```
[2/26/2020, 10:26:15 PM] [Config] [homebridge-tplink-smarthome] Error: ETIMEDOUT
```

Firewall:
Before restart
```
Feb 26 22:31:44 Pi kernel: [   25.068335] [UFW BLOCK] IN=eth0 OUT= MAC=ac:a5:32:24:81:5d:51:c7:bf:32:00:2b:08:00 SRC=192.168.1.46 DST=192.168.1.12 LEN=622 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=9999 DPT=35356 LEN=602
```

After restart
```
Feb 26 22:42:50 Pi kernel: [  660.069582] [UFW BLOCK] IN=eth0 OUT= MAC=ac:a5:32:24:81:5d:51:c7:bf:32:00:2b:08:00 SRC=192.168.1.46 DST=192.168.1.12 LEN=624 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=9999 DPT=56673 LEN=604
```