PM / devfreq: event: Add const keyword for devfreq_event_ops structure

This patch adds the const keyword for devfreq_event_ops structure
because the ops of devfreq_event_desc structure should not be changed
after initialization.

Cc: Myungjoo Ham <myungjoo.ham@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>