changed to int32 because of compiler error

[derDieDasJojo]

Compiling the ci_dev branch I got the following error:

/home/pi/romi-cidev/libromi/libromi/src/hal/I2C.cpp: In member function ‘uint8_t romi::I2C::compute_checksum()’:
/home/pi/romi-cidev/libromi/libromi/src/hal/I2C.cpp:87:29: error: conversion from ‘int’ to ‘uint16_t’ {aka ‘short unsigned int’} may change value [-Werror=conversion]
   87 |                         sum += buffer_[i];
      |                         ~~~~^~~~~~~~~~~~~

with the adjustments of this PR compiling was successfull. It should not have other side-effects because the sum is afterwards masked and converted to uint8