[csl] enable cca for CSL transmission (#11027)

This commit enables CCA for CSL transmission. CCA is expected for CSL
transmissions. The platform layer may skip CCA if mCsmaCaEnabled is not
set. This commit ensures mCsmaCaEnabled is correctly delivered to the
platform layer.

src/core/thread/csl_tx_scheduler.cpp

@@ -224,7 +224,7 @@ Mac::TxFrame *CslTxScheduler::HandleFrameRequest(Mac::TxFrames &aTxFrames)
     frame->SetTxDelay(txDelay);
     frame->SetTxDelayBaseTime(
         static_cast<uint32_t>(mCslTxNeighbor->GetLastRxTimestamp())); // Only LSB part of the time is required.
-    frame->SetCsmaCaEnabled(false);
+    frame->SetCsmaCaEnabled(true);
 
 exit:
     return frame;