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>
diff --git a/drivers/net/usb/hso.c b/drivers/net/usb/hso.c
index 191c1fa..d855cea 100644
--- a/drivers/net/usb/hso.c
+++ b/drivers/net/usb/hso.c
@@ -2253,6 +2253,7 @@
 
 	/* unlink and free TX URB */
 	usb_free_urb(serial->tx_urb);
+	kfree(serial->tx_buffer);
 	kfree(serial->tx_data);
 	tty_port_destroy(&serial->port);
 }