x86: Use printk_once()

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
LKML-Reference: <1249847649-11631-6-git-send-email-marcin.slusarz@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>