Revert "tcpip_send_msg_wait_sem() can return void now due to my last changes"

My brain was asleep

This reverts commit 475be665ff.

src/api/tcpip.c

@@ -314,8 +314,9 @@ tcpip_untimeout(sys_timeout_handler h, void *arg)
  * @param fn function to be called from TCPIP thread
  * @param apimsg argument to API function
  * @param sem semaphore to wait on
+ * @return ERR_OK if the function was called, another err_t if not
  */
-void
+err_t
 tcpip_send_msg_wait_sem(tcpip_callback_fn fn, void *apimsg, sys_sem_t* sem)
 {
 #if LWIP_TCPIP_CORE_LOCKING
@@ -323,6 +324,7 @@ tcpip_send_msg_wait_sem(tcpip_callback_fn fn, void *apimsg, sys_sem_t* sem)
   LOCK_TCPIP_CORE();
   fn(apimsg);
   UNLOCK_TCPIP_CORE();
+  return ERR_OK;
 #else /* LWIP_TCPIP_CORE_LOCKING */
   LWIP_ASSERT("semaphore not initialized", sys_sem_valid(sem));
 
@@ -337,6 +339,7 @@ tcpip_send_msg_wait_sem(tcpip_callback_fn fn, void *apimsg, sys_sem_t* sem)
   sys_mbox_post(&mbox, &TCPIP_MSG_VAR_REF(msg));
   sys_arch_sem_wait(sem, 0);
   TCPIP_MSG_VAR_FREE(msg);
+  return ERR_OK;
 #endif /* LWIP_TCPIP_CORE_LOCKING */
 }