Reduce duplicates of channel struct

Add/Fix custom clock limit defines (for YMF278B)

src/engine/platform/tx81z.h

@@ -20,8 +20,6 @@
 #ifndef _TX81Z_H
 #define _TX81Z_H
 #include "fmshared_OPM.h"
-#include "../macroInt.h"
-#include "../instrument.h"
 #include <queue>
 #include "sound/ymfm/ymfm_opz.h"
 
@@ -35,41 +33,12 @@ class DivPlatformTX81Z: public DivPlatformOPM {
       0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07
     };
 
-    struct Channel {
-      DivInstrumentFM state;
-      DivMacroInt std;
-      unsigned char freqH, freqL;
-      int freq, baseFreq, pitch, pitch2, note, ins;
-      signed char konCycles;
-      bool active, insChanged, freqChanged, keyOn, keyOff, inPorta, portaPause, furnacePCM, hardReset;
-      int vol, outVol;
+    struct Channel: public FMChannel {
       unsigned char chVolL, chVolR;
-      void macroInit(DivInstrument* which) {
-        std.init(which);
-        pitch2=0;
-      }
       Channel():
-        freqH(0),
-        freqL(0),
-        freq(0),
-        baseFreq(0),
-        pitch(0),
-        pitch2(0),
-        note(0),
-        ins(-1),
-        active(false),
-        insChanged(true),
-        freqChanged(false),
-        keyOn(false),
-        keyOff(false),
-        inPorta(false),
-        portaPause(false),
-        furnacePCM(false),
-        hardReset(false),
-        vol(0),
-        outVol(0),
-        chVolL(127),
-        chVolR(127) {}
+        FMChannel(),
+        chVolL(1),
+        chVolR(1) {}
     };
     Channel chan[8];
     DivDispatchOscBuffer* oscBuf[8];