hso: fix memory leak when device disconnects

In the disconnect path, tx_buffer should freed like tx_data to avoid
a memory leak when the device disconnects.
Signed-off-by: Olivier Sobrie <olivier@sobrie.be>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/hso.c

@@ -2253,6 +2253,7 @@ static void hso_serial_common_free(struct hso_serial *serial)
 
 	/* unlink and free TX URB */
 	usb_free_urb(serial->tx_urb);
+	kfree(serial->tx_buffer);
 	kfree(serial->tx_data);
 	tty_port_destroy(&serial->port);
 }