Fix writes >= 64KB

utility/SdFat.h

   /** \return SdVolume that contains this file. */
   SdVolume* volume(void) const {return vol_;}
   size_t write(uint8_t b);
-  size_t write(const void* buf, uint16_t nbyte);
+  size_t write(const void* buf, size_t nbyte);
   size_t write(const char* str);
   void write_P(PGM_P str);
   void writeln_P(PGM_P str);

utility/SdFile.cpp

  * for a read-only file, device is full, a corrupt file system or an I/O error.
  *
  */
-size_t SdFile::write(const void* buf, uint16_t nbyte) {
+size_t SdFile::write(const void* buf, size_t nbyte) {
   // convert void* to uint8_t*  -  must be before goto statements
   const uint8_t* src = reinterpret_cast<const uint8_t*>(buf);
 
   // number of bytes left to write  -  must be before goto statements
-  uint16_t nToWrite = nbyte;
+  size_t nToWrite = nbyte;
 
   // error if not a normal file or is read-only
   if (!isFile() || !(flags_ & O_WRITE)) goto writeErrorReturn;