tcp: reset conn->nrtx when ack received

Otherwise, when a long test triggers multiple timeout retransmissions,
the late timeout retransmissions are always delayed between 24 and 48 seconds

Signed-off-by: zhanghongyu <zhanghongyu@xiaomi.com>

net/tcp/tcp_input.c

@@ -773,6 +773,7 @@ static void tcp_input(FAR struct net_driver_s *dev, uint8_t domain,
                     tcp_getsequence(conn->sndseq), ackseq, unackseq,
                     (uint32_t)conn->tx_unacked);
               tcp_setsequence(conn->sndseq, ackseq);
+              conn->nrtx = 0;
             }
         }
 #endif