DiscIO: Allow converting from formats other than ISO and GCZ

The constant DESIRED_BUFFER_SIZE was determined by multiplying the
old hardcoded value 32 with the default GCZ block size 16 KiB.
Not sure if it actually is the best value, but it seems fine.

Source/Core/DiscIO/VolumeVerifier.cpp

@@ -1031,7 +1031,7 @@ void VolumeVerifier::SetUpHashing()
   else if (m_volume.GetVolumeType() == Platform::WiiDisc)
   {
     // Set up a DiscScrubber for checking whether blocks with errors are unused
-    m_scrubber.SetupScrub(&m_volume, VolumeWii::BLOCK_TOTAL_SIZE);
+    m_scrubber.SetupScrub(&m_volume);
   }
 
   std::sort(m_blocks.begin(), m_blocks.end(),