stream: Preserve enum class type in GetState()

Preserves the meaning/type-safetiness of the stream state instead of
making it an opaque u32. This makes it usable for other things outside
of the service HLE context.

src/audio_core/audio_renderer.h

@@ -170,7 +170,7 @@ public:
     u32 GetSampleRate() const;
     u32 GetSampleCount() const;
     u32 GetMixBufferCount() const;
-    u32 GetState() const;
+    Stream::State GetStreamState() const;
 
 private:
     class VoiceState;