Add mode 4 (force off) for the DMR OVCM flag.

DMRCSBK.cpp

@@ -1,5 +1,5 @@
 /*
- *   Copyright (C) 2015,2016,2020 by Jonathan Naylor G4KLX
+ *   Copyright (C) 2015,2016,2020,2021 by Jonathan Naylor G4KLX
  *   Copyright (C) 2019 by Patrick Maier DK5MP
  *
  *   This program is free software; you can redistribute it and/or modify
@@ -189,6 +189,12 @@ void CDMRCSBK::setOVCM(bool ovcm)
 	}
 }
 
+void CDMRCSBK::clearOVCM()
+{
+	if (getOVCM())
+		m_data[2U] ^= 0x04U;
+}
+
 bool CDMRCSBK::getGI() const
 {
 	return m_GI;