mtl/mxm: fix typo from opal_free_list_t update

ompi/mca/mtl/mxm/mtl_mxm_recv.c

@@ -188,8 +188,7 @@ int ompi_mtl_mxm_imrecv(struct mca_mtl_base_module_t* mtl,
         return OMPI_ERROR;
     }
 
-    msgp = (ompi_mtl_mxm_message_t *)
-        opal_free_list_wait (&mca_mtl_mxm_component.mxm_messages);
+    opal_free_list_return (&mca_mtl_mxm_component.mxm_messages, (opal_free_list_item_t *) msgp);
 
     ompi_message_return(*message);
     (*message) = MPI_MESSAGE_NULL;