Teach LoRaMacIsBusy return false if the MAC is stopped (#1326)

The previous version of LoRaMacIsBusy returned true when invoked while
the MAC is stopped. I believe this is incorrect. The MAC should not
indicate that is is busy while it is stopped.

Author: janakj

src/mac/LoRaMac.c

@@ -1522,6 +1522,11 @@ static void LoRaMacHandleIrqEvents( void )
 
 bool LoRaMacIsBusy( void )
 {
+    if( MacCtx.MacState == LORAMAC_STOPPED )
+    {
+        return false;
+    }
+
     if( LoRaMacRadioEvents.Events.RxProcessPending == 1 )
     {
         return true;