WiFi.disconnect() does not actually disconnect.

Need to replace it with the actual disconnect function.

http://esp-idf.readthedocs.io/en/latest/api-reference/wifi/esp_wifi.html#_CPPv219esp_wifi_disconnectv

```cpp
bool WiFiSTAClass::disconnect(bool wifioff)
{
    bool ret;
    ret = esp_wifi_disconnect() == ESP_OK;

    if(wifioff) {
        WiFi.enableSTA(false);
    }

    return ret;
}
```
