x86: Fix kernel-doc warnings

Signed-off-by: Wanpeng Li <liwp@linux.vnet.ibm.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Jason Wessel <jason.wessel@windriver.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Gavin Shan <shangw@linux.vnet.ibm.com>
Cc: Wanpeng Li <liwp.linux@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>