Don't read from disk when checking volume type

Should be faster than relying on the OS to cache the magic words.
Also gets rid of the odd recursive call in VolumeDirectory.

Source/Core/Core/VolumeHandler.cpp

@@ -77,10 +77,10 @@ bool IsValid()
 	return (g_pVolume != nullptr);
 }
 
-bool IsWii()
+bool IsWiiDisc()
 {
 	if (g_pVolume)
-		return IsVolumeWiiDisc(g_pVolume);
+		return g_pVolume->IsWiiDisc();
 
 	return false;
 }