serial: 8250_dw: Add valid clk pointer check

Commit ffc3ae6d "serial: 8250_dw: Enable runtime PM" introduced runtime
PM management, which enables/disables the clk without checking if the clk
is valid. However, this driver allows to be probed without a defined clk,
using clock-frequency, as a fallback.

Therefore, on platforms that are device tree probed using clock-frequency
instead of clk, we get an ugly NULL pointer dereference.

This patch fixes it by simply adding a check before accessing the clk api.
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_dw.c

@@ -338,6 +338,7 @@ static int dw8250_runtime_suspend(struct device *dev)
 {
 	struct dw8250_data *data = dev_get_drvdata(dev);
 
+	if (!IS_ERR(data->clk))
 		clk_disable_unprepare(data->clk);
 
 	return 0;
@@ -347,6 +348,7 @@ static int dw8250_runtime_resume(struct device *dev)
 {
 	struct dw8250_data *data = dev_get_drvdata(dev);
 
+	if (!IS_ERR(data->clk))
 		clk_prepare_enable(data->clk);
 
 	return 0;