net: zperf: Fix multicast data failure to receive server's AckFIN packet

In iperf2, for multicast data, if it is a client, it will not wait for
the server's AckFIN packet. Because the iperf2 server will not send an
AckFIN packet.
So in zperf_upload_fin(), an error will occur when the zperf client
waits for the server's AckFIN packet.
Multicast only send the negative sequence number packet and doesn't
wait for a server ack can fix this issue.

Signed-off-by: Gang Li <gang.li_1@nxp.com>