serial: serial-uclass: Use force parameter in stdio_deregister_dev()

On my x86 platform I've noticed, that calling dm_uninit() or the new
function dm_remove_devices_flags() does not remove the desired device at
all. Debugging showed, that the serial uclass returns -EPERM in
serial_pre_remove(). This patch sets the force parameter when calling
stdio_deregister_dev() resulting in a removal of the device.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

drivers/serial/serial-uclass.c

@@ -349,7 +349,7 @@ static int serial_pre_remove(struct udevice *dev)
 #if CONFIG_IS_ENABLED(SYS_STDIO_DEREGISTER)
 	struct serial_dev_priv *upriv = dev_get_uclass_priv(dev);
 
-	if (stdio_deregister_dev(upriv->sdev, 0))
+	if (stdio_deregister_dev(upriv->sdev, true))
 		return -EPERM;
 #endif