Update libretro.cpp

Fixes CDDA and ADPCM options being swapped and volume number increase being opposite what you expect. That solves two parts of https://github.com/libretro/mednafen-libretro/issues/23, but getting the CDPSG volume setting working is beyond my capability.
Merge pull request #52 from Awakened0/patch-1

Partial Fix for Core Volume Settings