如何修复隐式有符号转无符号转换（Warning 373）问题

我知道这个问题已被问过几次，但之前的答案不够清晰。该警告似乎与编译器将char类型提升为unsigned char有关……我在下方函数的两处str[numbytes-i] = r + 0x30;位置收到该警告，正确的解决方法是什么？

void usitoh( UINT16 val, UINT8 numbytes, char *str )       //extern
{
    UINT8 i = 0;
    UINT8 r = 0;
        
    //Convert the Base 10 Number to Base 16
    //Starting with LSB (i=1) and work to the MSB
    for (i=1; i <= numbytes; i++) 
    {
        r = val % 16;       //Get the MOD of the number
                            //The MOD of the number represents the Hex value
        val = val / 16;     //Get the Quotient of the number
        
        //Load the value into the string and 
        //add an offset to generate ASCII values
        // '0' = 0x30, 'A' = 0x41
        if (r <= 9) //Values < A
        {
            //For values 0-9 the ASCII offset is 0x30 
            //This produces and offset to get to chars 0,1,2,3,4,5,6,7,8,9
            //Warning 373 issue
            str[numbytes-i] = r + 0x30;
        }
        else        //Values > 9
        {
            //The MOD of the number represents the Hex value
            //For values 0-9 the ASCII offset is 0x41 to get to "A" and - 0x0A to account 0-9 numbers
            //This produces and offset to get to chars A,B,C,D,E,F
            //Warning 373 issue
            str[numbytes-i] = r + (0x41 - 0x0A);
        }        
    }//END of FOR Loop
}//END of "usitoh"

更新 #

感谢大家的建议和指导。@chux的替代代码、@Persixty的简洁方案都很实用。按建议进行类型转换后问题已解决。

补充代码背景：我在硬件重设计项目中接手此代码，客户称其为良好起点，实际却一团糟。代码用MPLAB、XC8编译器编写，存在超500条警告、注释问题、编码规范差等问题，经了解是多人修改的遗留产物，大写类型名是原作者自定义的。

内容的提问来源于stack exchange，提问作者Gerald Eberhardt