The reference says:

     5.2.3. SYN-RECEIVED
     ...
     Departures
       - A CLOSE request is made by the user.  Create a packet with
         FIN set.  Send it and go to the FIN-WAIT state.

Add this missing step.

Probably not a real usecase for barebox anyway as there is no user
triggered close.

Signed-off-by: Aleksander Morgado <aleksander@aleksander.es>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>