Add a switch to disable Management Frame Protection (802.11w)

In the case of WPA3 encryption Management Frame Protection is enabled
by default in OpenWrt.

But in some cases, it causes trouble with particular devices that
fails to connect to WiFi Access Point - see:
https://forum.turris.cz/t/turris-omnia-wifi-health/15704/15

src/common/WiFiSettings/WiFiSettings.js

@@ -59,6 +59,10 @@ function prepDataToSubmit(formData) {
 
         if (!device.guest_wifi.enabled)
             formData.devices[idx].guest_wifi = { enabled: false };
+
+        if (device.encryption === "WPA2") {
+            delete formData.devices[idx].ieee80211w_disabled;
+        }
     });
     return formData;
 }