PM / devfreq: fix missing check of return value in devfreq_add_device()

devm_kzalloc() could fail, so insert a check of its return value. And
if it fails, returns -ENOMEM.

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