Fix type of nrFiles

Firmware/cardreader.h

@@ -135,7 +135,7 @@ private:
 
   bool autostart_stilltocheck; //the sd start is delayed, because otherwise the serial cannot answer fast enought to make contact with the hostsoftware.
   
-  int16_t nrFiles; //counter for the files in the current directory and recycled as position counter for getting the nrFiles'th name in the directory.
+  uint16_t nrFiles; //counter for the files in the current directory and recycled as position counter for getting the nrFiles'th name in the directory.
 
   bool diveSubfolder (const char *&fileName);
   void lsDive(const char *prepend, SdFile parent, const char * const match=NULL, LsAction lsAction = LS_GetFilename, ls_param lsParams = ls_param());