PM / Domain: Return 0 on error from of_genpd_opp_to_performance_state()

of_genpd_opp_to_performance_state() should return 0 on errors, as its
doc comment describes. While it follows that mostly, it returns a
negative error number on one of the failures.

Fix that.

Fixes: 6e41766a "PM / Domain: Implement of_genpd_opp_to_performance_state()"
Reported-by: Rajendra Nayak <rnayak@codeaurora.org>
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>