net: zperf: Improve UDP multicast upload

UDP multicast uploads did not produce zperf-compatible server stats,
so the client waited for and tried to decode a non-existent reply,
causing “Unexpected response flags” and misleading
“LAST PACKET NOT RECEIVED!!!” output.

This change treats multicast as client-only: after sending
the negative-id FIN it does not wait for or decode server stats,
Instead transmits the FIN a few times to help receivers detect test end.
The shell prints a clear “Statistics (client only)” summary for multicast
and suppresses the error when client stats are valid.

Unicast behavior is unchanged.

Signed-off-by: Kapil Bhatt <kapil.bhatt@nordicsemi.no>