Convert char [] of ascii hex digits to unsigned long long (uint64_t)

I've been spending some time on this, and just can't figure out how to convert a character string of hexadecimal digits into an unsigned long long (uint64_t) without using any library routines but only using the shift operator. I need this for a kernel mode driver. I've read this SO post and tried the code given, but it does not produce correct results.

Basically, what I'm trying to do is convert a string like so:

char buffer[] = "FFFFFFFF06749565";

into an unsigned long long (unint64_t) as follows:

0xFFFFFFFF06749565

ntoskrnl.dll export _strtoui64 so you can use this code:

bool fromstring(_In_ PCSTR str, _Out_ ULONG64& value)
{
    value = _strtoui64(str, const_cast<char**>(&str), 16);
    return !*str;
}

if want do this direct (from my view no sense, but as demo only)

bool fromstring(_In_ PCSTR str, _Out_ ULONG64& value)
{
    ULONG64 v = 0, n = 17;

    do 
    {
        ULONG c = *str++;

        if (!c)
        {
            value = v;
            return true;
        }

        v <<= 4;

        if (c - '0' <= '9' - '0')
        {
            v |= c - '0';
            continue;
        }
        
        if (c - 'A' <= 'F' - 'A')
        {
            v |= c - 'A' + 10;
            continue;
        }
        
        if (c - 'a' <= 'f' - 'a')
        {
            v |= c - 'a' + 10;
            continue;
        }

        return false;

    } while (--n);

    return false;
}