serial: of_serial: use devm_clk_get() instead of clk_get()

The probe method of this driver calls clk_get(), but clk_put() is
missing from the remove callback.

Using the managed clk function is easier than fixing other parts.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/of_serial.c

@@ -67,7 +67,7 @@ static int of_platform_serial_setup(struct platform_device *ofdev,
 	if (of_property_read_u32(np, "clock-frequency", &clk)) {
 
 		/* Get clk rate through clk driver if present */
-		info->clk = clk_get(&ofdev->dev, NULL);
+		info->clk = devm_clk_get(&ofdev->dev, NULL);
 		if (IS_ERR(info->clk)) {
 			dev_warn(&ofdev->dev,
 				"clk or clock-frequency not defined\n");