A few constants where they belong

Marlin/SdFatConfig.h

@@ -113,8 +113,10 @@ uint8_t const SOFT_SPI_SCK_PIN = 13;
  */
 /** Number of VFAT entries used. Every entry has 13 UTF-16 characters */
 #define MAX_VFAT_ENTRIES (2)
+/** Number of UTF-16 characters per entry */
+#define FILENAME_LENGTH 13
 /** Total size of the buffer used to store the long filenames */
-#define LONG_FILENAME_LENGTH (13*MAX_VFAT_ENTRIES+1)
+#define LONG_FILENAME_LENGTH (FILENAME_LENGTH*MAX_VFAT_ENTRIES+1)
 #endif  // SdFatConfig_h