PM / devfreq: Change return type of devfreq_set_freq_table()

This patch changes the return type of devfreq_set_freq_table()
from 'void' to 'int' in order to check whether it fails or not.

And This patch just removes the 'devfreq' prefix and the description
of function. Because the helper functions are only used by the devfreq.

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>