improve qmk documentation

Update esp8266_tencent_firmware.c

Update ec20.c

Update bc35_28_95_lwm2m.c

Update m26.c

Update m5310a.c

Update sim7600ce.c

Update sim800a.c

Update bc35_28_95.c

Update air724.c

Fixed stack overflow bug in bc26.c

    if (!str) 
    {
        return -1;
    }

    if (!str) 
    {
        return -1;
    }

    if (!str) 
    {
        return -1;
    }

    if (!str) 
    {
        return -1;
    }

    if (!str) 
    {
        return -1;
    }