GBA SIO: Fix unconnected SIOCNT for multi mode (fixes #1105)

CHANGES

@@ -37,6 +37,7 @@ Bugfixes:
  - Core: Fix audio sync breaking when interrupted
  - Qt: Improve FPS timer stability
  - GBA Serialize: Fix loading channel 3 volume (fixes mgba.io/i/1107)
+ - GBA SIO: Fix unconnected SIOCNT for multi mode (fixes mgba.io/i/1105)
 Misc:
  - GBA Timer: Use global cycles for timers
  - GBA: Extend oddly-sized ROMs to full address space (fixes mgba.io/i/722)

src/gba/sio.c

@@ -163,6 +163,9 @@ void GBASIOWriteSIOCNT(struct GBASIO* sio, uint16_t value) {
 				value &= ~0x0080;
 			}
 			break;
+		case SIO_MULTI:
+			value |= 0xC;
+			break;
 		default:
 			// TODO
 			break;