Add a comment explaining that mce_chrdev_ops is intentionally
writable.

[ Impact: comment only ]
Signed-off-by: NH. Peter Anvin <hpa@zytor.com>
Signed-off-by: NHidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Signed-off-by: NH. Peter Anvin <hpa@zytor.com>