quoted

So maybe more like this:

static inline bool i2c_check_quirks(struct i2c_adapter *adap, u64 quirks)
{
if (!adap->quirks)
return false;
return (adap->quirks->flags & quirks) == quirks;
}

Should I use bool (like in your snippet) or int (like i2c_check_functionality) as return type?

I'd use bool, given that the result is a boolean value. It's semantically
more clear this way.