PM / devfreq: fix missing unlock on error in exynos4_busfreq_pm_notifier_event()

Add the missing unlock before return from function
exynos4_busfreq_pm_notifier_event() in the error
handling case.

This issue introduced by commit 8fa938
(PM / devfreq: exynos4_bus: honor RCU lock usage)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
[Resolved path conflicts by MyungJoo Ham]
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>

drivers/devfreq/exynos/exynos4_bus.c

@@ -974,6 +974,7 @@ static int exynos4_busfreq_pm_notifier_event(struct notifier_block *this,
 			rcu_read_unlock();
 			dev_err(data->dev, "%s: unable to find a min freq\n",
 				__func__);
+			mutex_unlock(&data->lock);
 			return PTR_ERR(opp);
 		}
 		new_oppinfo.rate = opp_get_freq(opp);