PM / Domains: Fix return value of of_genpd_opp_to_performance_state()

of_genpd_opp_to_performance_state() should return 0 for errors, but the
dummy routine isn't doing that. Fix it.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>