Scripts/Spell: fix Mixology value bonus

based on http://forum.serevo.it/index.php/topic,11211.0/wap2.html researches

maybe there is a better solution ... let me know :)

Closes  #2172
is there a chance to simplify this code?
```c++
DoEffectCalcAmount += AuraEffectCalcAmountFn(spell_gen_mixology_bonus_AuraScript::CalculateAmount, EFFECT_ALL, SPELL_AURA_ANY);
```