x86: setup: issue a null command after enabling A20 via KBC

Apparently, DOS and possibly other legacy operating systems issued a
null command to the keyboard controller after toggling A20,
specifically "pulse output pins" with no output pins specified.  This
was presumably done for synchronization reasons.  This has made it
into at least the UHCI spec, and it has been found to cause
compatibility problems when "legacy USB" is enabled (which it almost
always is) to not have this byte sent.

It is *NOT* clear if any of these compatibility problems has any
effect on Linux.  However, for maximum compatibility, issue this null
command after togging A20 through the KBC.
Signed-off-by: NH. Peter Anvin <hpa@zytor.com>

arch/x86/boot/a20.c

 /* -*- linux-c -*- ------------------------------------------------------- *
  *
  *   Copyright (C) 1991, 1992 Linus Torvalds
- *   Copyright 2007 rPath, Inc. - All Rights Reserved
+ *   Copyright 2007-2008 rPath, Inc. - All Rights Reserved
  *
  *   This file is part of the Linux kernel, and is made available under
  *   the terms of the GNU General Public License version 2.
@@ -95,6 +95,9 @@ static void enable_a20_kbc(void)
 
 	outb(0xdf, 0x60);	/* A20 on */
 	empty_8042();
+
+	outb(0xff, 0x64);	/* Null command, but UHCI wants it */
+	empty_8042();
 }
 
 static void enable_a20_fast(void)