All ACKs without data must be built in the same way from the input
message:
     <SN=received AN><AN=received SN+1 modulo 2><CTL=ACK>

The code was actually doing this instead:
     <SN=0><AN=SN><CTL=ACK>

This change fixes how the retransmissions are ACK-ed by the peer, and
would have affected the barebox<->bbremote interactions.

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