Address review comments

src/core/hle/kernel/mutex.cpp

@@ -109,7 +109,7 @@ void Mutex::UpdatePriority() {
     if (!holding_thread)
         return;
 
-    u32 best_priority = THREADPRIO_LOWEST;
+    u32 best_priority = ThreadPrioLowest;
     for (auto& waiter : GetWaitingThreads()) {
         if (waiter->current_priority < best_priority)
             best_priority = waiter->current_priority;