Fix read >= 64KB

(attention, untested)

utility/SdFat.h

     uint8_t b;
     return read(&b, 1) == 1 ? b : -1;
   }
-  int16_t read(void* buf, uint16_t nbyte);
+  int32_t read(void* buf, size_t nbyte);
   int8_t readDir(dir_t* dir);
   static uint8_t remove(SdFile* dirFile, const char* fileName);
   uint8_t remove(void);

utility/SdFile.cpp

  * read() called before a file has been opened, corrupt file system
  * or an I/O error occurred.
  */
-int16_t SdFile::read(void* buf, uint16_t nbyte) {
+int32_t SdFile::read(void* buf, size_t nbyte) {
   uint8_t* dst = reinterpret_cast<uint8_t*>(buf);
 
   // error if not open or write only
   if (nbyte > (fileSize_ - curPosition_)) nbyte = fileSize_ - curPosition_;
 
   // amount left to read
-  uint16_t toRead = nbyte;
+  uint32_t toRead = nbyte;
   while (toRead > 0) {
     uint32_t block;  // raw device block number
     uint16_t offset = curPosition_ & 0X1FF;  // offset in block
       }
       block = vol_->clusterStartBlock(curCluster_) + blockOfCluster;
     }
-    uint16_t n = toRead;
+    int32_t n = toRead;
 
     // amount to be read from current block
     if (n > (512 - offset)) n = 512 - offset;