Reduce duplicates of channel struct

Add/Fix custom clock limit defines (for YMF278B)

src/engine/platform/msm6295.h

@@ -26,28 +26,17 @@
 
 class DivPlatformMSM6295: public DivDispatch, public vgsound_emu_mem_intf {
   protected:
-    struct Channel {
-      int note, ins;
-      signed char konCycles;
-      bool active, insChanged, freqChanged, keyOn, keyOff, furnacePCM, hardReset;
-      int vol, outVol;
+    struct Channel: public SharedChannel, public SharedChannelVolume<int> {
+      bool furnacePCM;
       int sample;
       DivMacroInt std;
       void macroInit(DivInstrument* which) {
         std.init(which);
       }
       Channel():
-        note(0),
-        ins(-1),
-        active(false),
-        insChanged(true),
-        freqChanged(false),
-        keyOn(false),
-        keyOff(false),
+        SharedChannel(),
+        SharedChannelVolume<int>(8),
         furnacePCM(false),
-        hardReset(false),
-        vol(8),
-        outVol(8),
         sample(-1) {}
     };
     Channel chan[4];