SDL_n3dsaudio.c: separate mixer locks from audio device locks

(cherry picked from commit 62266dbd4fa79090025317a71473eb764b2e1abe)

(SDL3 audio backends don't have the LockDevice interfaces, so this just
ended up being a comment.)

src/audio/n3ds/SDL_n3dsaudio.c

@@ -32,6 +32,7 @@
 static dspHookCookie dsp_hook;
 static SDL_AudioDevice *audio_device;
 
+// fully local functions related to the wavebufs / DSP, not the same as the `device->lock` SDL_Mutex!
 static SDL_INLINE void contextLock(SDL_AudioDevice *device)
 {
     LightLock_Lock(&device->hidden->lock);