[CPUFREQ] mark hotplug notifier callback as __cpuinit

The notifier_block is already __cpuinitdata, thereby allowing us to safely
mark the callback function as __cpuinit also, thereby saving space when
HOTPLUG_CPU=n.

Signed-off-by: Satyam Sharma <satyam@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dave Jones <davej@redhat.com>