dm: serial: Adjust serial_setconfig() to use proper API

All driver-model functions should have a device as the first parameter.
Update this function accordingly.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
diff --git a/drivers/serial/serial-uclass.c b/drivers/serial/serial-uclass.c
index 81f1067..669c82f 100644
--- a/drivers/serial/serial-uclass.c
+++ b/drivers/serial/serial-uclass.c
@@ -305,16 +305,13 @@
 	return 0;
 }
 
-int serial_setconfig(uint config)
+int serial_setconfig(struct udevice *dev, uint config)
 {
 	struct dm_serial_ops *ops;
 
-	if (!gd->cur_serial_dev)
-		return 0;
-
-	ops = serial_get_ops(gd->cur_serial_dev);
+	ops = serial_get_ops(dev);
 	if (ops->setconfig)
-		return ops->setconfig(gd->cur_serial_dev, config);
+		return ops->setconfig(dev, config);
 
 	return 0;
 }