Use new MUS_NONE constant in sound.c

src/sound.c

@@ -264,7 +264,7 @@ void FadeInNewBGM(u16 songNum, u8 speed)
 {
     if (gDisableMusic)
         songNum = 0;
-    if (songNum == 0xFFFF)
+    if (songNum == MUS_NONE)
         songNum = 0;
     m4aSongNumStart(songNum);
     m4aMPlayImmInit(&gMPlayInfo_BGM);
@@ -561,7 +561,7 @@ void PlayBGM(u16 songNum)
 {
     if (gDisableMusic)
         songNum = 0;
-    if (songNum == 0xFFFF)
+    if (songNum == MUS_NONE)
         songNum = 0;
     m4aSongNumStart(songNum);
 }