PM / devfreq: Clean up the devfreq instance name in sysfs attr

The sysfs attr interface used eithere 'df' or 'devfreq' for devfreq instance
name. In order to keep the consistency and to improve the readabilty,
unify the instance name as 'df'. Add add the missing conditional statement
to prevent the fault.

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>