staging/ft1000: fix sparse warning

sparse warns about having 0 assigned to a pointer,
fix it up by using NULL.

The following sparse warning is fixed

drivers/staging/ft1000/ft1000-usb/ft1000_debug.c:170:52: warning: Using plain integer as NULL pointer
Signed-off-by: Devendra Naga <devendra.aaru@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/ft1000/ft1000-usb/ft1000_debug.c

@@ -167,7 +167,7 @@ int ft1000_create_dev(struct ft1000_device *dev)
 		goto fail;
 	}
 
-	dir = debugfs_create_dir(info->DeviceName, 0);
+	dir = debugfs_create_dir(info->DeviceName, NULL);
 	if (IS_ERR(dir)) {
 		result = PTR_ERR(dir);
 		goto debug_dir_fail;