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/DiscIO/VolumeDirectory.h

@@ -45,6 +45,7 @@ public:
 	u32 GetFSTSize() const override;
 
 	std::string GetApploaderDate() const override;
+	bool IsWiiDisc() const override;
 
 	ECountry GetCountry() const override;
 
@@ -85,6 +86,8 @@ private:
 
 	u32 m_totalNameSize;
 
+	bool m_is_wii;
+
 	// GameCube has no shift, Wii has 2 bit shift
 	u32 m_addressShift;