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: 6e41766a6a50 "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>

drivers/base/power/domain.c

@@ -2431,7 +2431,8 @@ unsigned int of_genpd_opp_to_performance_state(struct device *dev,
 
 	opp = of_dev_pm_opp_find_required_opp(&genpd->dev, opp_node);
 	if (IS_ERR(opp)) {
-		state = PTR_ERR(opp);
+		dev_err(dev, "Failed to find required OPP: %ld\n",
+			PTR_ERR(opp));
 		goto unlock;
 	}