platform/x86: dell-uart-backlight: fix serdev race

The dell_uart_bl_serdev_probe() function calls devm_serdev_device_open()
before setting the client ops via serdev_device_set_client_ops(). This
ordering can trigger a NULL pointer dereference in the serdev controller's
receive_buf handler, as it assumes serdev->ops is valid when
SERPORT_ACTIVE is set.

This is similar to the issue fixed in commit 5e700b384ec1
("platform/chrome: cros_ec_uart: properly fix race condition") where
devm_serdev_device_open() was called before fully initializing the
device.

Fix the race by ensuring client ops are set before enabling the port via
devm_serdev_device_open().

Note, serdev_device_set_baudrate() and serdev_device_set_flow_control()
calls should be after the devm_serdev_device_open() call.

Fixes: 484bae9e4d6a ("platform/x86: Add new Dell UART backlight driver")
Signed-off-by: Chenyuan Yang <[e-mail]>
Reviewed-by: Hans de Goede <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Reviewed-by: Ilpo Järvinen <[e-mail]>
Signed-off-by: Ilpo Järvinen <[e-mail]>

drivers/platform/x86/dell/dell-uart-backlight.c

···
 	init_waitqueue_head(&dell_bl->wait_queue);
 	dell_bl->dev = dev;
 
+	serdev_device_set_drvdata(serdev, dell_bl);
+	serdev_device_set_client_ops(serdev, &dell_uart_bl_serdev_ops);
+
 	ret = devm_serdev_device_open(dev, serdev);
 	if (ret)
 		return dev_err_probe(dev, ret, "opening UART device\n");
···
 	/* 9600 bps, no flow control, these are the default but set them to be sure */
 	serdev_device_set_baudrate(serdev, 9600);
 	serdev_device_set_flow_control(serdev, false);
-	serdev_device_set_drvdata(serdev, dell_bl);
-	serdev_device_set_client_ops(serdev, &dell_uart_bl_serdev_ops);
 
 	get_version[0] = DELL_SOF(GET_CMD_LEN);
 	get_version[1] = CMD_GET_VERSION;