PM / devfreq: Fix compiler warnings for CONFIG_PM_DEVFREQ unset

Fix compiler warnings generated when devfreq is not enabled
(CONFIG_PM_DEVFREQ is not set).

Signed-off-by: Rajagopal Venkat <rajagopal.venkat@linaro.org>
Acked-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>