usb: dwc3: keystone: check return value

Function devm_clk_get() returns an ERR_PTR when it fails. However, in
function kdwc3_probe(), its return value is not checked, which may
result in a bad memory access bug. This patch fixes the bug.
Signed-off-by: NPan Bian <bianpan2016@163.com>
Signed-off-by: NFelipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/dwc3/dwc3-keystone.c

@@ -107,6 +107,10 @@ static int kdwc3_probe(struct platform_device *pdev)
 		return PTR_ERR(kdwc->usbss);
 
 	kdwc->clk = devm_clk_get(kdwc->dev, "usb");
+	if (IS_ERR(kdwc->clk)) {
+		dev_err(kdwc->dev, "unable to get usb clock\n");
+		return PTR_ERR(kdwc->clk);
+	}
 
 	error = clk_prepare_enable(kdwc->clk);
 	if (error < 0) {