Fixed hostname on ESP32

[1Prototype1]

This commit fixes the hostname not getting applied on ESP32 since WiFi.mode() was called before setting the hostname.

Explanation

Taking a look at the WiFi library,
you can see setHostname method calls set_esp_netif_hostname which sets the default_hostname variable with the provided hostname.
This variable is read only by get_esp_netif_hostname which gets called in mode and only when the mode is changed to WIFI_MODE_STA ref

TL;DR

1. Call WiFi.setHostname() to set internal variable default_hostname with desired hostname.
2. Call WiFi.mode() with parameter WIFI_STA which will apply the hostname.

Tested and working on ESP32 (Arduino-ESP32 v2.0.17) and no breaking changes found on ESP8266.