sceIo : Only write d_private for memory stick

Custom BGM still working good and Alien vs predator okay to boot now. Fixes https://github.com/hrydgard/ppsspp/issues/5856

![npjh50465_00000](https://cloud.githubusercontent.com/assets/3000282/2689291/afccd1a4-c2f6-11e3-8cee-62e97cdf51e8.jpg)