Accurate loading times for Wii games

This is a continuation of [PR 1634](https://github.com/dolphin-emu/dolphin/pull/1617). WII_IPC_HLE_Device_DI now uses all the timing accuracy improvements that DVDInterface has gotten throughout this year. Also, the fast disc speed option now works for Wii games. The second and third commits might cause problems in games that rely on specific IPC timings, so testing is needed.
I'm not really sure either. The original code gave me the feeling that IOS doesn't start executing a command until the previous one is done. If that's the case, += seems to be the right thing to do. If that's not the case, why are we trying to make replies happen in order anyway? It seems unlikely that IOS would execute multiple commands at once and then add arbitrary delays just to get output in a specific order...