Emulate GameCube memory card speeds

Fixes http://code.google.com/p/dolphin-emu/issues/detail?id=2284

Attempts to emulate GameCube memory card transfer speeds.
I'm not a big fan of this, could you instead make that a virtual function like: `virtual bool NeedsSpecificTiming() const { return false; }` and override it in the memcard class?

(With a better name - I don't really know what it does)