GBA SIO: Improve SIO Normal dummy driver (fixes #520)

CHANGES

@@ -163,6 +163,7 @@ Bugfixes:
  - GB Video: Fix LYC regression
  - Qt: Fix translation initialization (fixes mgba.io/i/776)
  - PSP2: Use custom localtime_r since newlib version is broken (fixes mgba.io/i/560)
+ - GBA SIO: Improve SIO Normal dummy driver (fixes mgba.io/i/520)
 Misc:
  - Qt: Add language selector
  - GBA Timer: Improve accuracy of timers

src/gba/sio.c

@@ -155,11 +155,13 @@ void GBASIOWriteSIOCNT(struct GBASIO* sio, uint16_t value) {
 		case SIO_NORMAL_8:
 		case SIO_NORMAL_32:
 			value |= 0x0004;
-			if ((value & 0x4080) == 0x4080) {
-				// TODO: Test this on hardware to see if this is correct
-				GBARaiseIRQ(sio->p, IRQ_SIO);
+			if ((value & 0x0081) == 0x0081) {
+				if (value & 0x4000) {
+					// TODO: Test this on hardware to see if this is correct
+					GBARaiseIRQ(sio->p, IRQ_SIO);
+				}
+				value &= ~0x0080;
 			}
-			value &= ~0x0080;
 			break;
 		default:
 			// TODO