x86: Fix typo in kexec register clearing

Fixes a typo in register clearing code. Thanks to PaX Team for fixing
this originally, and James Troup for pointing it out.

Signed-off-by: Kees Cook <keescook@chromium.org>
Link: http://lkml.kernel.org/r/20130605184718.GA8396@www.outflux.net
Cc: <stable@vger.kernel.org> v2.6.30+
Cc: PaX Team <pageexec@freemail.hu>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>