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
its the other way round, isnt it? I mean, just look at the description of Load: 
https://github.com/TrinityCore/TrinityCore/blob/master/src/server/game/Spells/SpellScript.h#L120