[pcap] fix pcap callback for TX frames (openthread#10678)

This commit fixes the pcap callback for TX frames:

* Report the tx frame on each tx started callback, so that
  retransmissions can be captured.
* Set the RSSI to be invalid for TX frames.

src/core/mac/sub_mac.cpp

@@ -473,11 +473,6 @@ void SubMac::BeginTransmit(void)
 
     SetState(kStateTransmit);
 
-    if (mPcapCallback.IsSet())
-    {
-        mPcapCallback.Invoke(&mTransmitFrame, true);
-    }
-
     error = Get<Radio>().Transmit(mTransmitFrame);
 
     if (error == kErrorInvalidState && mTransmitFrame.mInfo.mTxInfo.mTxDelay > 0)
@@ -497,6 +492,11 @@ void SubMac::BeginTransmit(void)
 
 void SubMac::HandleTransmitStarted(TxFrame &aFrame)
 {
+    if (mPcapCallback.IsSet())
+    {
+        mPcapCallback.Invoke(&aFrame, true);
+    }
+
     if (ShouldHandleAckTimeout() && aFrame.GetAckRequest())
     {
         StartTimer(kAckTimeout);

src/ncp/ncp_base_mtd.cpp

@@ -4600,9 +4600,10 @@ void NcpBase::HandlePcapFrame(const otRadioFrame *aFrame, bool aIsTx)
     SuccessOrExit(mEncoder.WriteData(aFrame->mPsdu, aFrame->mLength));
 
     // Append metadata (rssi, etc)
-    SuccessOrExit(mEncoder.WriteInt8(aFrame->mInfo.mRxInfo.mRssi)); // RSSI
-    SuccessOrExit(mEncoder.WriteInt8(-128));                        // Noise floor (Currently unused)
-    SuccessOrExit(mEncoder.WriteUint16(flags));                     // Flags
+    SuccessOrExit(
+        mEncoder.WriteInt8((aIsTx ? static_cast<int8_t>(OT_RADIO_RSSI_INVALID) : aFrame->mInfo.mRxInfo.mRssi))); // RSSI
+    SuccessOrExit(mEncoder.WriteInt8(-128));    // Noise floor (Currently unused)
+    SuccessOrExit(mEncoder.WriteUint16(flags)); // Flags
 
     SuccessOrExit(mEncoder.OpenStruct()); // PHY-data
     // Empty for now