[mac] always emit OT_CHANGED_THREAD_CHANNEL on channel change (#3560)

src/core/mac/mac.cpp

@@ -364,6 +364,7 @@ otError Mac::SetPanChannel(uint8_t aChannel)
 
     mPanChannel = aChannel;
     mCcaSuccessRateTracker.Reset();
+    GetNotifier().Signal(OT_CHANGED_THREAD_CHANNEL);
 
     VerifyOrExit(!mRadioChannelAcquisitionId);
 
@@ -371,8 +372,6 @@ otError Mac::SetPanChannel(uint8_t aChannel)
 
     UpdateIdleMode();
 
-    GetNotifier().Signal(OT_CHANGED_THREAD_CHANNEL);
-
 exit:
     return error;
 }