x86: fix microcode driver newly spewing warnings

Jeff Garzik reported this WARN_ON() noise:

> Kernel: 2.6.30-rc1-00306-g8371f87
> Hardware: ICH10 x86-64
>
> This is a regression from 2.6.29.  Microcode spews the following WARNING
> multiple times during boot:
>
> ------------[ cut here ]------------
> WARNING: at fs/sysfs/group.c:138 sysfs_remove_group+0xeb/0xf0()
> Hardware name:         sysfs group ffffffffa0209700 not found for
>  kobject 'cpu0'

Keep sysfs files around for cpus even when we failed to locate
microcode for them at the moment of module loading. The appropriate
microcode firmware can become available later on.

Signed-off-by: Ingo Molnar <mingo@elte.hu>