samples: modules: nanopb: fix encode buffer size
samples: modules: nanopb: fix encode buffer size

A bug was introduced when encoding/decoding moved to separate
functions where sizeof no longer had the correct argument.

The size of the encode buffer is passed to the encode function.

Signed-off-by: Pieter De Gendt <b5b50e3ea5dc0014327690ec78e146f3b45ee870@basalte.be>

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