samples: net: zperf: Fix TCP uploader
samples: net: zperf: Fix TCP uploader

Try to construct a proper iperf client header to the payload
as the server expects that. It is not sure if this is needed
or correct as there is no proper documentation for the iPerf2
protocol.

Fixes #14665

Signed-off-by: Jukka Rissanen <f9f077d1da4c7aa947f9864fa5a28c3588f6a45d@linux.intel.com>

GiulianoFranchetto/zephyr,ldts/zephyr,Vudentz/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,finikorg/zephyr,Vudentz/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr,finikorg/zephyr,ldts/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,galak/zephyr