iface-modem-3gpp-ussd: fix warnings with -Wswitch-default

mm-iface-modem-3gpp-ussd.c: In function ‘ensure_enabled’:
  mm-iface-modem-3gpp-ussd.c:59:5: error: switch missing default case [-Werror=switch-default]
     59 |     switch (modem_state) {
        |     ^~~~~~
  mm-iface-modem-3gpp-ussd.c: In function ‘interface_disabling_step’:
  mm-iface-modem-3gpp-ussd.c:565:5: error: switch missing default case [-Werror=switch-default]
    565 |     switch (ctx->step) {
        |     ^~~~~~
  mm-iface-modem-3gpp-ussd.c: In function ‘interface_enabling_step’:
  mm-iface-modem-3gpp-ussd.c:709:5: error: switch missing default case [-Werror=switch-default]
    709 |     switch (ctx->step) {
        |     ^~~~~~
  mm-iface-modem-3gpp-ussd.c: In function ‘interface_initialization_step’:
  mm-iface-modem-3gpp-ussd.c:831:5: error: switch missing default case [-Werror=switch-default]
    831 |     switch (ctx->step) {
        |     ^~~~~~

src/mm-iface-modem-3gpp-ussd.c

@@ -89,6 +89,10 @@ ensure_enabled (MMBaseModem *self,
     case MM_MODEM_STATE_CONNECTING:
     case MM_MODEM_STATE_CONNECTED:
         break;
+
+    default:
+        g_assert_not_reached ();
+        return FALSE;
     }
 
     return TRUE;
@@ -588,6 +592,9 @@ interface_disabling_step (GTask *task)
         g_task_return_boolean (task, TRUE);
         g_object_unref (task);
         return;
+
+    default:
+        break;
     }
 
     g_assert_not_reached ();
@@ -732,6 +739,9 @@ interface_enabling_step (GTask *task)
         g_task_return_boolean (task, TRUE);
         g_object_unref (task);
         return;
+
+    default:
+        break;
     }
 
     g_assert_not_reached ();
@@ -907,6 +917,9 @@ interface_initialization_step (GTask *task)
         g_task_return_boolean (task, TRUE);
         g_object_unref (task);
         return;
+
+    default:
+        break;
     }
 
     g_assert_not_reached ();