Clean up the compiler warnings

[ceremcem]

Following warnings are produced when compiled with -Wconversion -std=c99 -pedantic -Wall -Wextra flags:

/home/ceremcem/embedded/gcc/gcc-arm-none-eabi-10-2020-q4-major/bin//arm-none-eabi-gcc -c -mcpu=cortex-m4 -mthumb -mfpu=fpv4-sp-d16 -mfloat-abi=hard -DUSE_HAL_DRIVER -DSTM32F407xx -ICore/Inc -IDrivers/STM32F4xx_HAL_Driver/Inc -IDrivers/STM32F4xx_HAL_Driver/Inc/Legacy -IDrivers/CMSIS/Device/ST/STM32F4xx/Include -IDrivers/CMSIS/Include -Imodbus_lib -O0 -Wconversion -std=c99 -pedantic -Wall -Wextra -Wall -fdata-sections -ffunction-sections -Wextra -g -gdwarf-2 -MMD -MP -MF"build/modbus_lib.d" -Wa,-a,-ad,-alms=build/modbus_lib.lst modbus_lib/modbus_lib.c -o build/modbus_lib.o
modbus_lib/modbus_lib.c: In function 'modbus_lib_end_of_telegram':
modbus_lib/modbus_lib.c:81:43: warning: conversion from 'int' to 'uint8_t' {aka 'unsigned char'} may change value [-Wconversion]
   81 |             outgoing_telegram[oindex++] = count.value * 2;                      // byte count
      |                                           ^~~~~
modbus_lib/modbus_lib.c:84:74: warning: conversion from 'int' to 'uint16_t' {aka 'short unsigned int'} may change value [-Wconversion]
   84 |                 res.value = modbus_lib_read_handler(start_addr.value + i + MB_ADDRESS_HOLDING_REGISTER_OFFSET);
modbus_lib/modbus_lib.c: In function 'modbus_lib_send_error':
modbus_lib/modbus_lib.c:130:13: warning: conversion from 'uint16_t' {aka 'short unsigned int'} to 'unsigned char' may change value [-Wconversion]
  130 |             config->address,
      |             ^~~~~~
modbus_lib/modbus_lib.c:132:13: warning: conversion from 'int' to 'unsigned char' may change value [-Wconversion]
  132 |             error_code
      |             ^~~~~~~~~~
modbus_lib/modbus_lib.c:141:12: warning: unsigned conversion from 'int' to 'uint16_t' {aka 'short unsigned int'} changes value from '-1' to '65535' [-Wsign-conversion]
  141 |     return -1;
      |