plugins/ksys573_da_offset: Use "pre-start" callback.

The names of the callbacks are misleading - "pre-start" is actually
start, "start" is actually reset.  Eventually the callback code needs to
be changed and the names can be made less misleading.

plugins/ksys573_da_offset/init.lua

@@ -150,7 +150,7 @@ function ksys573_da_offset.startplugin()
 		return menu
 	end
 
-	emu.register_start(
+	emu.register_prestart(
 			function ()
 				if not manager.machine.devices[":k573dio"] then
 					counter_offset = nil