workqueue: use __cpuinit instead of __devinit for cpu callbacks

For workqueue hotplug callbacks, it makes less sense to use __devinit
which discards the memory after boot if !HOTPLUG.  __cpuinit, which
discards the memory after boot if !HOTPLUG_CPU fits better.

tj: Updated description.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>