PM / devfreq: Fix governor_store()

Writing the currently set governor into sysfs currently
seems to fail.
Fix this by setting the return code to zero before
leaving governor_store().

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>