Always wait until complete frame has been transmitted

mbus/mbus-serial.c

@@ -213,6 +213,11 @@ mbus_serial_send_frame(mbus_serial_handle *handle, mbus_frame *frame)
         return -1;
     }
     
+    //
+    // wait until complete frame has been transmitted
+    //
+    tcdrain(handle->fd);
+
     return 0;
 }