PM / devfreq: Reduce log severity for informative message

Suppose devfreq_update_status() failure in devfreq_set_target() is not a
critical error, reduces the log severity.

Signed-off-by: Tzung-Bi Shih <tzungbi@google.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>