usb: dwc3-generic: Return early when there is no child node

The current error check for device_find_first_child is not working as
expected, the documentation for device_find_first_child mention:

  @devp: Returns first child device, or NULL if none
  Return: 0

Change to return early when there is no child node to avoid any possible
null pointer dereference.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>

drivers/usb/dwc3/dwc3-generic.c

@@ -558,9 +558,9 @@ int dwc3_glue_probe(struct udevice *dev)
 			return ret;
 	}
 
-	ret = device_find_first_child(dev, &child);
-	if (ret)
-		return ret;
+	device_find_first_child(dev, &child);
+	if (!child)
+		return 0;
 
 	if (glue->clks.count == 0) {
 		ret = dwc3_glue_clk_init(child, glue);