[TTY] Register tty devclass before use.

Register the tty devclass with sysfs before tty drivers initialise -
sysfs requires structures to be registered before use.  This is
required for the previous serial csets, as well as any drivers which
are initialising using __initcall() or module_init().

drivers/char/tty_io.c

@@ -2235,14 +2235,19 @@ struct device_class tty_devclass = {
 };
 EXPORT_SYMBOL(tty_devclass);
 
+static int __init tty_devclass_init(void)
+{
+	return devclass_register(&tty_devclass);
+}
+
+postcore_initcall(tty_devclass_init);
+
 /*
  * Ok, now we can initialize the rest of the tty devices and can count
  * on memory allocations, interrupts etc..
  */
 void __init tty_init(void)
 {
-	devclass_register(&tty_devclass);
-
 	/*
 	 * dev_tty_driver and dev_console_driver are actually magic
 	 * devices which get redirected at open time.  Nevertheless,