The reference says:

    If the SYN flag was set but the ACK was not set then the other
    end of the connection has executed an active open also.
    Acknowledge the SYN, choose your MDL, and send:
        <SN=0><AN=received SN+1 modulo 2><CTL=SYN, ACK><LENGTH=MDL>
    Go to the SYN-RECEIVED state without any further processing.

Add this missing step.

This fix isn't related to any barebox<->bbremote interaction issue,
because the case where both ends start an active connection doesn't
apply in this case.

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