PM: Update function header for dev_pm_get_subsys_data()

The commit "PM: Make dev_pm_get_subsys_data() always return 0 on success"
changed the return value from dev_pm_get_subsys_data(). Let's update the
comment in the function header to reflect this change as well.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>