Thermal: Tidy up error handling in powerclamp_init

This patch
 * adds missing kfree() for cpu_clamping_mask
 * adds return value checking for alloc_percpu()
 * unregister hotcpu notifier in exit path

Signed-off-by: Durgadoss R <durgadoss.r@intel.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>