overlays: Fix cpu usage in low detail mode

rpcs3/Emu/RSX/Overlays/overlay_perf_metrics.cpp

@@ -453,7 +453,7 @@ namespace rsx
 					}
 					case detail_level::low:
 					{
-						if (m_cpu_usage < 0.)
+						if (m_detail == detail_level::low) // otherwise already acquired in medium
 							m_cpu_usage = static_cast<f32>(m_cpu_stats.get_usage());
 
 						[[fallthrough]];