Fixes #5483

This only applies to "base game" spells.
When adding an AutoCalc spell with TES:CS its cost is computed and
stored inside game files. This stored cost was being used by OpenMW and
the actual cost was never recomputed at runtime whereas Morrowind.exe
discards the stored cost.
While this worked fine in vanilla, mods can update AutoCalc spell
effects without ever updating the stored cost.

The formula was mostly there already but there was a few differences,
namely a 1 second offset in duration.