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.

Wind Waker J has been tested and confirmed working with this by JMC47
I was expecting that.  The device_type is overidden because according to this:

https://github.com/TotalNerd/dolphin/blob/memcard-emu/Source/Core/Core/HW/EXI_Device.h#L23

It's supposed to be converted internally, but because of https://github.com/TotalNerd/dolphin/blob/memcard-emu/Source/Core/Core/HW/EXI_Device.cpp#L141 the device remains a EXIDEVICE_MEMORYCARDFOLDER.

The change was made because of https://github.com/TotalNerd/dolphin/blob/memcard-emu/Source/Core/Core/HW/EXI_Channel.cpp#L140