Fix return value for ecrt_master_send_ext().

include/ecrt.h

@@ -1110,6 +1110,7 @@ EC_PUBLIC_API int ecrt_master_receive(
  * This method has to be called in the send callback function passed via
  * ecrt_master_callbacks() to allow the sending of non-application datagrams.
  * \return Zero on success, otherwise negative error code.
+ * \retval -EAGAIN Lock could not be acquired, try again later.
  */
 int ecrt_master_send_ext(
         ec_master_t *master /**< EtherCAT master. */

master/master.c

@@ -2534,7 +2534,7 @@ int ecrt_master_send_ext(ec_master_t *master)
     ec_datagram_t *datagram, *next;
 
     if (down_trylock(&master->ext_queue_sem))
-        return;
+        return -EAGAIN;
 
     list_for_each_entry_safe(datagram, next, &master->ext_datagram_queue,
             ext_queue) {