wi-fi network connection: Fixed error message when entering the form

- When selecting an existing WiFi on the "scan" page, the user will be redirected to the "WiFi add" form.
  This form did not contain a WiFi capable Interface as default selection, for this reason the user get a
  invalid form message when entering the form.

plinth/modules/networks/networks.py

@@ -360,7 +360,9 @@ def add_wifi(request, ssid=None):
     form_data = None
 
     if ssid:
+        device = network.get_first_wifi_device()
         form_data = {'name': ssid,
+                     'interface': device,
                      'zone': 'external',
                      'ssid': ssid,
                      'mode': 'infrastructure',

plinth/network.py

@@ -76,6 +76,17 @@ def get_interface_list(device_type):
     return interfaces
 
 
+def get_first_wifi_device():
+    """Get a list of network interface available on the system."""
+    interface = "empty"
+    for device in nm.Client.new(None).get_devices():
+        if device.get_device_type() == nm.DeviceType.WIFI:
+            interface = device.get_iface()
+            return interface
+
+    return interface
+
+
 def get_ip_from_device(devicename):
     """
     Get the first ip address from the network interface.