hso: add missing cancel_work_sync in disconnect()

For hso serial devices, two cancel_work_sync were missing in the
disconnect method.

Signed-off-by: Olivier Sobrie <olivier@sobrie.be>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/hso.c

@@ -3106,6 +3106,8 @@ static void hso_free_interface(struct usb_interface *interface)
 			mutex_lock(&serial->parent->mutex);
 			serial->parent->usb_gone = 1;
 			mutex_unlock(&serial->parent->mutex);
+			cancel_work_sync(&serial_table[i]->async_put_intf);
+			cancel_work_sync(&serial_table[i]->async_get_intf);
 			hso_serial_tty_unregister(serial);
 			kref_put(&serial_table[i]->ref, hso_serial_ref_free);
 			set_serial_by_index(i, NULL);