PM / devfreq: fix mem leak in devfreq_add_device()

'devfreq' is malloced in devfreq_add_device() and should be freed in
the error handling cases, otherwise it will cause memory leak.

Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>