platform/surface: clean up a variable in surface_dtx_read()

The "&client->ddev->lock" and "&ddev->lock" are the same thing.  Let's
use "&ddev->lock" consistently.
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Maximilian Luz <luzmaximilian@gmail.com>
Link: https://lore.kernel.org/r/YF3TgCcpcCYl3a//@mwandaSigned-off-by: Hans de Goede <hdegoede@redhat.com>

drivers/platform/surface/surface_dtx.c

@@ -487,7 +487,7 @@ static ssize_t surface_dtx_read(struct file *file, char __user *buf, size_t coun
 			if (status < 0)
 				return status;
 
-			if (down_read_killable(&client->ddev->lock))
+			if (down_read_killable(&ddev->lock))
 				return -ERESTARTSYS;
 
 			/* Need to check that we're not shut down again. */