USB: serial: fix tty cleanup-op kernel-doc

The tty cleanup operation is called with a struct tty as its sole
parameter.

Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/usb-serial.c

@@ -288,7 +288,7 @@ static void serial_close(struct tty_struct *tty, struct file *filp)
 
 /**
  * serial_cleanup - free resources post close/hangup
- * @port: port to free up
+ * @tty: tty to clean up
  *
  * Do the resource freeing and refcount dropping for the port.
  * Avoid freeing the console.