pkg/lwip: adapt to API change of netdev_new_api

Since #21012 a netdev in the new API
can return > 0 directly in netdev_driver_t::send() to indicate the
driver is naturally synchronous and has already completed the
transmission.

The adaption of lwIP to that API change contained a bug: It handled the
case after the thread is already blocked waiting for the signal that
is never going to arrive. This is now fixed.

pkg/lwip/contrib/netdev/lwip_netdev.c

@@ -298,14 +298,16 @@ static err_t _common_link_output(struct netif *netif, netdev_t *netdev, iolist_t
         return ERR_IF;
     }
 
-    /* block until TX completion is signaled from IRQ */
-    thread_flags_wait_any(THREAD_FLAG_LWIP_TX_DONE);
+    /* `res > 0` means transmission already completed according to API contract.
+     * ==> only waiting when res == 0 */
+    if (res == 0) {
+        /* block until TX completion is signaled from IRQ */
+        thread_flags_wait_any(THREAD_FLAG_LWIP_TX_DONE);
 
-    irq_state = irq_disable();
-    compat_netif->thread_doing_tx = NULL;
-    irq_restore(irq_state);
+        irq_state = irq_disable();
+        compat_netif->thread_doing_tx = NULL;
+        irq_restore(irq_state);
 
-    if (res == 0) {
         /* async send */
         while (-EAGAIN == (res = netdev->driver->confirm_send(netdev, NULL))) {
             /* this should not happen, as the driver really only should emit the