OPP: fix error code in dev_pm_opp_set_config()

This is an error path so set the error code.  Smatch complains about the
current code:

    drivers/opp/core.c:2660 dev_pm_opp_set_config()
    error: uninitialized symbol 'ret'.

Fixes: e37440e ("OPP: Call dev_pm_opp_set_opp() for required OPPs")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/3f3660af-4ea0-4a89-b3b7-58de7b16d7a5@stanley.mountain
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

Author: Dan Carpenter

drivers/opp/core.c

@@ -2630,8 +2630,10 @@ int dev_pm_opp_set_config(struct device *dev, struct dev_pm_opp_config *config)
 
 	/* Attach genpds */
 	if (config->genpd_names) {
-		if (config->required_devs)
+		if (config->required_devs) {
+			ret = -EINVAL;
 			goto err;
+		}
 
 		ret = _opp_attach_genpd(opp_table, dev, config->genpd_names,
 					config->virt_devs);