Core/Spells: fix cast time talent bonus of channeled spells

This fixes the cast time talent bonus of channeled spells, including Improved Succubus (https://github.com/TrinityCore/TrinityCore/issues/1543)
The point @joschiwald is trying to make is not mods being applied twice to the same value but one spell taking two charges of a spellmod (if its a proc with charges)