SWDEV-521647 - Fix tracking of hw_event (#206)

- When a command may possibly have two packets(like device heap
  initializer), and if there is no signal on the main kernel packet the
tracking was broken as it marked HW event of the command as the first
packet signal.
- Make sure if no completion signal is attached to the second packet
  then clear the HW event for the command.

rocclr/platform/command.hpp

@@ -213,7 +213,7 @@ class Event : public RuntimeObject {
   //! Returns the callback for this event
   const CallBackEntry* Callback() const { return callbacks_; }
 
-  // Saves HW event, associated with the current command
+  //! Saves HW event, associated with the current command
   void SetHwEvent(void* hw_event) { hw_event_ = hw_event; }
 
   //! Returns HW event, associated with the current command