MSU1: speed up bulk PCM file scanning by some games

Some games, notably ALTTP Randomizer, perform a full scan of all PCM tracks on startup. After adding MSU1 SRAM checks, this led to a full SRAM check per track scanned (because it was considered a safe opportunity to always do an SRAM check on track change), resulting in 7+ seconds of one-time startup time for the game which was considered unacceptable.
SRAM check is now relaxed to at most once per second on track change; startup time is reduced to about 600ms.

Author: mrehkopf

src/msu1.c

@@ -96,7 +96,7 @@ void prepare_audio_track(uint16_t msu_track, uint32_t audio_offset) {
   f_close(&msuaudio);
   msu_audio_usage = MSU_IDLE;
   if(is_msu_free_to_save()) {
-    msu_savecheck(1);
+    msu_savecheck(0);
   }
   snprintf(suffix, sizeof(suffix), "-%d.pcm", msu_track);
   strcpy((char*)file_buf, (char*)file_lfn);