when src_i.ack stayed HIGH after src_o.cyc when LOW (which is clearly a bug),
the ack_cnt was still counting acks (which should confirm all STABed data
words). Thus, when new frame came (src_o.cyc HIGH again), the ack_cnt was
completly lost and weired things happened. So, now the src_i.ack is ignored
when src_o.cyc is LOW.