Reduce duplicates of channel struct

Add/Fix custom clock limit defines (for YMF278B)

src/engine/platform/msm6258.h

@@ -26,44 +26,18 @@
 
 class DivPlatformMSM6258: public DivDispatch {
   protected:
-    struct Channel {
-      unsigned char freqH, freqL;
-      int freq, baseFreq, pitch, pitch2, portaPauseFreq, note, ins;
-      unsigned char psgMode, autoEnvNum, autoEnvDen;
-      signed char konCycles;
-      bool active, insChanged, freqChanged, keyOn, keyOff, portaPause, inPorta, furnacePCM, hardReset;
-      int vol, outVol;
+    struct Channel: public SharedChannel, public SharedChannelVolume<int> {
+      bool furnacePCM;
       int sample;
       unsigned char pan;
       DivMacroInt std;
       void macroInit(DivInstrument* which) {
         std.init(which);
-        pitch2=0;
       }
       Channel():
-        freqH(0),
-        freqL(0),
-        freq(0),
-        baseFreq(0),
-        pitch(0),
-        pitch2(0),
-        portaPauseFreq(0),
-        note(0),
-        ins(-1),
-        psgMode(1),
-        autoEnvNum(0),
-        autoEnvDen(0),
-        active(false),
-        insChanged(true),
-        freqChanged(false),
-        keyOn(false),
-        keyOff(false),
-        portaPause(false),
-        inPorta(false),
+        SharedChannel(),
+        SharedChannelVolume<int>(8),
         furnacePCM(false),
-        hardReset(false),
-        vol(0),
-        outVol(15),
         sample(-1),
         pan(3) {}
     };