UDP portion of the echo_client sends 1 packet with the sample data.
(TCP can send chunks of it depending on the response of the send()
function.) Not every network interface can send a UDP packet large
enough to handle the size of the sample data.
Let's make sure to account for the network interface MTU when deciding
the amount of sample data to send.
Fixes: https://github.com/zephyrproject-rtos/zephyr/issues/22447
Signed-off-by: Michael Scott <mike@foundries.io>
2c7e420a8e