saturn - hide unavailable ports depending on multitap

One slight issue is that the bulitin smpc reset is always "player 13", but that's not enough of an issue for me to care

src/BizHawk.Emulation.Cores/Consoles/Sega/Saturn/Saturnus.cs

@@ -57,5 +57,21 @@ namespace BizHawk.Emulation.Cores.Consoles.Sega.Saturn
 		protected override ISet<string> NonSyncSettingNames { get; } = new HashSet<string>
 		{
 		};
+
+		protected override HashSet<string> ComputeHiddenPorts()
+		{
+			var devCount = 12;
+			if (SettingsQuery("ss.input.sport1.multitap") != "1")
+				devCount -= 5;
+			if (SettingsQuery("ss.input.sport2.multitap") != "1")
+				devCount -= 5;
+			var ret = new HashSet<string>();
+			for (var i = 1; i <= 12; i++)
+			{
+				if (i > devCount)
+					ret.Add($"port{i}");
+			}
+			return ret;
+		}
 	}
 }