Fix Wii disc partitions

This is magumagu's [dvd-low-open-partition](https://github.com/magumagu/dolphin/tree/dvd-low-open-partition) rebased on top of a recent version of master, requested by JMC47. Fixes Masterpieces in Super Smash Bros. Brawl.
I'm not sure.  The old code assumed the TMD was always exactly 0x208 bytes, and I don't know if it can be any other size.