Actions workflow: error: conversion

[vanvught]

src/artnet/artnetrdmcontroller.cpp:210:79: error: conversion from 'int' to 'uint8_t' {aka 'unsigned char'} may change value [-Werror=conversion]
[435](https://github.com/vanvught/rpidmx512/actions/runs/3612916020/jobs/6088333398#step:6:436)
  210 |  s_rdmMessage.message_length = static_cast<uint8_t>(RDM_MESSAGE_MINIMUM_SIZE) + s_rdmMessage.param_data_length;
[436](https://github.com/vanvught/rpidmx512/actions/runs/3612916020/jobs/6088333398#step:6:437)

  |                                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~