Exposed SDCard interface to the public, to USB MSD is able to read/write sectors on the SD card

Marlin/src/sd/cardreader.h

@@ -86,6 +86,7 @@ public:
   FORCE_INLINE uint8_t percentDone() { return (isFileOpen() && filesize) ? sdpos / ((filesize + 99) / 100) : 0; }
   FORCE_INLINE char* getWorkDirName() { workDir.getFilename(filename); return filename; }
 
+  Sd2Card& getSd2Card() { return card; }
 public:
   bool saving, logging, sdprinting, cardOK, filenameIsDir;
   char filename[FILENAME_LENGTH], longFilename[LONG_FILENAME_LENGTH];