drivers: wifi: esp32: fix possible interface null pointer

esp32_wifi_dev_init() currently starts Wi-Fi stack before
the interface is properly configured, which happens in function
esp32_wifi_init(). This can trigger a ESP32_WIFI_EVENT_STA_START event
before interface initialization, causing a crash.
Moving esp_wifi_start() to esp32_wifi_init() will guarantee that
this won't happen.

Signed-off-by: Sylvio Alves <sylvio.alves@espressif.com>