revert part of 3a41b52c to respect http method's purpose

ajax/networking/get_netcfg.php

@@ -3,7 +3,7 @@
 require '../../includes/csrf.php';
 require_once '../../includes/config.php';
 
-$interface = $_POST['iface'];
+$interface = $_GET['iface'];
 
 if (isset($interface)) {
     // fetch dnsmasq.conf settings for interface

app/js/custom.js

@@ -179,7 +179,7 @@ Option toggles are set dynamically depending on the loaded configuration
 */
 function loadInterfaceDHCPSelect() {
     var strInterface = $('#cbxdhcpiface').val();
-    $.post('ajax/networking/get_netcfg.php',{iface:strInterface},function(data){
+    $.get('ajax/networking/get_netcfg.php?iface='+strInterface,function(data){
         jsonData = JSON.parse(data);
         $('#dhcp-iface')[0].checked = jsonData.DHCPEnabled;
         $('#txtipaddress').val(jsonData.StaticIP);