SPU: Stub out external volume registers

If nothing else, it'll avoid spamming the log.

src/core/save_state_version.h

@@ -2,4 +2,4 @@
 #include "types.h"
 
 static constexpr u32 SAVE_STATE_MAGIC = 0x43435544;
-static constexpr u32 SAVE_STATE_VERSION = 12;
+static constexpr u32 SAVE_STATE_VERSION = 13;

src/core/spu.cpp

@@ -40,6 +40,8 @@ void SPU::Reset()
   m_main_volume_right = {};
   m_cd_audio_volume_left = 0;
   m_cd_audio_volume_right = 0;
+  m_external_volume_left = 0;
+  m_external_volume_right = 0;
   m_key_on_register = 0;
   m_key_off_register = 0;
   m_endx_register = 0;
@@ -93,6 +95,8 @@ bool SPU::DoState(StateWrapper& sw)
   sw.DoPOD(&m_main_volume_right);
   sw.Do(&m_cd_audio_volume_left);
   sw.Do(&m_cd_audio_volume_right);
+  sw.Do(&m_external_volume_left);
+  sw.Do(&m_external_volume_right);
   sw.Do(&m_key_on_register);
   sw.Do(&m_key_off_register);
   sw.Do(&m_endx_register);
@@ -219,6 +223,12 @@ u16 SPU::ReadRegister(u32 offset)
     case 0x1F801DB2 - SPU_BASE:
       return m_cd_audio_volume_right;
 
+    case 0x1F801DB4 - SPU_BASE:
+      return m_external_volume_left;
+
+    case 0x1F801DB6 - SPU_BASE:
+      return m_external_volume_right;
+
     case 0x1F801DB8 - SPU_BASE:
       m_tick_event->InvokeEarly();
       return m_main_volume_left.current_level;
@@ -441,6 +451,22 @@ void SPU::WriteRegister(u32 offset, u16 value)
       m_tick_event->InvokeEarly();
       m_cd_audio_volume_right = value;
     }
+    break;
+
+    case 0x1F801DB4 - SPU_BASE:
+    {
+      // External volumes aren't used, so don't bother syncing.
+      Log_DebugPrintf("SPU left external volume register <- 0x%04X", ZeroExtend32(value));
+      m_external_volume_left = value;
+    }
+    break;
+
+    case 0x1F801DB6 - SPU_BASE:
+    {
+      // External volumes aren't used, so don't bother syncing.
+      Log_DebugPrintf("SPU right external volume register <- 0x%04X", ZeroExtend32(value));
+      m_external_volume_right = value;
+    }
     break;
 
       // read-only registers

src/core/spu.h

@@ -399,6 +399,9 @@ private:
   s16 m_cd_audio_volume_left = 0;
   s16 m_cd_audio_volume_right = 0;
 
+  s16 m_external_volume_left = 0;
+  s16 m_external_volume_right = 0;
+
   u32 m_key_on_register = 0;
   u32 m_key_off_register = 0;
   u32 m_endx_register = 0;