[mac] add retry flag support for direct messages (#4421)

src/core/mac/mac.cpp

@@ -956,6 +956,7 @@ void Mac::BeginTransmit(void)
     TxFrame &sendFrame             = mSubMac.GetTransmitFrame();
 
     VerifyOrExit(mEnabled, error = OT_ERROR_ABORT);
+    sendFrame.SetIsARetransmission(false);
 
     switch (mOperation)
     {

src/core/mac/sub_mac.cpp

@@ -370,6 +370,7 @@ void SubMac::HandleTransmitDone(TxFrame &aFrame, RxFrame *aAckFrame, otError aEr
     if (shouldRetx)
     {
         mTransmitRetries++;
+        aFrame.SetIsARetransmission(true);
         StartCsmaBackoff();
         ExitNow();
     }