net: stmmac: fix missing unlock on error in stmmac_suspend()

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

Fixes: 5ec55823 ("net: stmmac: add clocks management for gmac driver")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/stmicro/stmmac/stmmac_main.c

@@ -5964,8 +5964,10 @@ int stmmac_suspend(struct device *dev)
 		/* Disable clock in case of PWM is off */
 		clk_disable_unprepare(priv->plat->clk_ptp_ref);
 		ret = pm_runtime_force_suspend(dev);
-		if (ret)
+		if (ret) {
+			mutex_unlock(&priv->lock);
 			return ret;
+		}
 	}
 
 	mutex_unlock(&priv->lock);