tty: of_serial: fix return value check in of_platform_serial_setup()

In case of error, the function clk_get() returns ERR_PTR()
and never returns NULL. The NULL test in the return value
check should be replaced with IS_ERR().

dpatch engine is used to auto generate this patch.
(https://github.com/weiyj/dpatch)
Signed-off-by: NWei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/of_serial.c

@@ -66,10 +66,10 @@ static int __devinit of_platform_serial_setup(struct platform_device *ofdev,
 
 		/* Get clk rate through clk driver if present */
 		info->clk = clk_get(&ofdev->dev, NULL);
-		if (info->clk == NULL) {
+		if (IS_ERR(info->clk)) {
 			dev_warn(&ofdev->dev,
 				"clk or clock-frequency not defined\n");
-			return -ENODEV;
+			return PTR_ERR(info->clk);
 		}
 
 		clk_prepare_enable(info->clk);