net: tcp: if buffer is TCP_FIN increment send_ack by 1

Now that the TCP_FIN block is after a potential packet_received()
tcp->send_ack should be appropriately set to the last sequence
processed.

In the case of a TCP_FIN buffer, we should advance it by 1 or else
the destination will continue to retry to send the last block.

Change-Id: I9c97d35a87ad6cc1a50f928b237780bff4cd2877
Signed-off-by: Michael Scott <michael.scott@linaro.org>