Reduce duplicates of channel struct

Add/Fix custom clock limit defines (for YMF278B)

src/engine/platform/genesisext.cpp

@@ -709,7 +709,7 @@ void DivPlatformGenesisExt::reset() {
   DivPlatformGenesis::reset();
 
   for (int i=0; i<4; i++) {
-    opChan[i]=DivPlatformGenesisExt::OpChannel();
+    opChan[i]=DivPlatformOPN::OPNOpChannelStereo();
     opChan[i].std.setEngine(parent);
     opChan[i].vol=127;
     opChan[i].outVol=127;