bdk: fatfs: improve fat read/write

- Convert access from min cluster size to block size
  Also allow read/writes to less than cluster size
- In case of intercluster access throw an error
- Do not error on zero size

On reads buffer still needs to be block (instead of cluster) aligned.
On writes, buffer still needs to be readable out of bounds.

bdk/libs/fatfs/ff.h

@@ -264,8 +264,10 @@ FRESULT f_open (FIL* fp, const TCHAR* path, BYTE mode);				/* Open or create a f
 FRESULT f_close (FIL* fp);											/* Close an open file object */
 FRESULT f_read (FIL* fp, void* buff, UINT btr, UINT* br);			/* Read data from the file */
 FRESULT f_write (FIL* fp, const void* buff, UINT btw, UINT* bw);	/* Write data to the file */
+#if FF_FASTFS /* buff needs to be block aligned. Intercluster data access is not supported. */
 FRESULT f_read_fast (FIL* fp, const void* buff, UINT btr);			/* Fast read data from the file */
 FRESULT f_write_fast (FIL* fp, const void* buff, UINT btw);         /* Fast write data to the file */
+#endif
 FRESULT f_lseek (FIL* fp, FSIZE_t ofs);								/* Move file pointer of the file object */
 FRESULT f_truncate (FIL* fp);										/* Truncate the file */
 FRESULT f_sync (FIL* fp);											/* Flush cached data of the writing file */
@@ -287,7 +289,9 @@ FRESULT f_getfree (const TCHAR* path, DWORD* nclst, FATFS** fatfs);	/* Get numbe
 FRESULT f_getlabel (const TCHAR* path, TCHAR* label, DWORD* vsn);	/* Get volume label */
 FRESULT f_setlabel (const TCHAR* label);							/* Set volume label */
 FRESULT f_forward (FIL* fp, UINT(*func)(const BYTE*,UINT), UINT btf, UINT* bf);	/* Forward data to the stream */
+#if FF_FASTFS
 DWORD  *f_expand_cltbl (FIL* fp, UINT tblsz, FSIZE_t ofs);			/* Expand file and populate cluster table */
+#endif
 FRESULT f_expand (FIL* fp, FSIZE_t fsz, BYTE opt);					/* Allocate a contiguous block to the file */
 FRESULT f_mount (FATFS* fs, const TCHAR* path, BYTE opt);			/* Mount/Unmount a logical drive */
 FRESULT f_mkfs (const TCHAR* path, BYTE opt, DWORD au, void* work, UINT len);	/* Create a FAT volume */