net: zperf: Fix upload ping timeout error

Fixes remote address for ping before upload. This caused the ping in zperf
upload to timeout as shown in the following output:

```
uart:~$ zperf udp upload 2001:db8::2 5001 10 50 1M
Remote port is 5001
Connecting to 2001:db8::2
Duration:       10.00 s
Packet size:    50 bytes
Rate:           1000 kbps
Starting...
ping 2001:db8::2 timeout
Rate:           1.00 Mbps
Packet duration 390 us
```

Fixes: https://github.com/zephyrproject-rtos/zephyr/issues/68674



Signed-off-by: Ayush Singh <ayushdevel1325@gmail.com>