FDS: Update IRQ based on latest hardware notes

- IRQ updates based on latest notes. Verified with test rom.

https://www.nesdev.org/wiki/Family_Computer_Disk_System#IRQ_control_($4022)

Discussion and test rom file:
https://forums.nesdev.org/viewtopic.php?f=3&t=16507
test rom: https://forums.nesdev.org/viewtopic.php?p=205052#p205052

- Puff Puff Golf is still problematic and unplayable. added notes for
  it.

- Fix typo for a previous commit.

src/fds.cpp

@@ -223,14 +223,23 @@ void FCEU_FDSSelect(void)
 	FCEU_DispMessage("Disk %d Side %c Selected", 0, SelectDisk >> 1, (SelectDisk & 1) ? 'B' : 'A');
 }
 
-#define IRQ_Repeat  (IRQa & 0x01)
-#define IRQ_Enabled (IRQa & 0x02)
+#define IRQ_Repeat  0x01
+#define IRQ_Enabled 0x02
 
 static void FDSFix(int a) {
-	if ((IRQa & IRQ_Enabled) && IRQCount) {
+	if (IRQa & IRQ_Enabled) {
 		IRQCount -= a;
 		if (IRQCount <= 0) {
 			IRQCount = IRQLatch;
+			/* Puff Puff Golf notes:
+			Game freezes while music playing ingame after inserting Disk Side B.
+			IRQ is usually fired at scanline 169 and 183 for music to work.
+			
+			At some point after inserting disk B, an IRQ is fired at scanline 174 which
+			will just freeze game while music plays.
+			
+			If you ignore triggering IRQ altogether, game plays but no music
+			*/
 			X6502_IRQBegin(FCEU_IQEXT);
 			if (!(IRQa & IRQ_Repeat)) {
 				IRQa &= ~IRQ_Enabled;
@@ -574,21 +583,30 @@ void FDSSoundReset(void) {
 static DECLFW(FDSWrite) {
 	switch (A) {
 	case 0x4020:
-		X6502_IRQEnd(FCEU_IQEXT);
 		IRQLatch &= 0xFF00;
 		IRQLatch |= V;
 		break;
 	case 0x4021:
-		X6502_IRQEnd(FCEU_IQEXT);
 		IRQLatch &= 0xFF;
 		IRQLatch |= V << 8;
 		break;
 	case 0x4022:
-		X6502_IRQEnd(FCEU_IQEXT);
-		IRQCount = IRQLatch;
-		IRQa = V & 3;
+		if (FDSRegs[3] & 1) {
+			IRQa = V & 0x03;
+			if (IRQa & IRQ_Enabled) {
+				IRQCount = IRQLatch;
+			} else {
+				X6502_IRQEnd(FCEU_IQEXT);
+			}
+		}
+		break;
+	case 0x4023:
+		if (!(V & 0x01)) {
+			IRQa &= ~IRQ_Enabled;
+			X6502_IRQEnd(FCEU_IQEXT);
+			X6502_IRQEnd(FCEU_IQEXT2);
+		}
 		break;
-	case 0x4023: break;
 	case 0x4024:
 		if (mapperFDS_diskinsert && ~mapperFDS_control & 0x04) {