PM / devfreq: refactor set_target frequency function

The refactoring is needed for the new client in devfreq: suspend.
To avoid code duplication, move it to the new local function
devfreq_set_target.

Suggested-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Suggested-by: Chanwoo Choi <cw00.choi@samsung.com>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Lukasz Luba <l.luba@partner.samsung.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>