fat: replace LINEAR_PREFETCH_SIZE with PREFETCH_BLOCKS

Currently in do_fat_read() when reading FAT sectors, we have to divide down
LINEAR_PREFETCH_SIZE by the sector size, whereas it's defined as 2 sectors
worth of bytes. In order to avoid redundant multiplication/division, introduce
#define PREFETCH_BLOCKS instead of #define LINEAR_PREFETCH_SIZE.

Signed-off-by: Sergei Shtylyov <sshtylyov@mvista.com>
diff --git a/include/fat.h b/include/fat.h
index c2465d2..15dbf10 100644
--- a/include/fat.h
+++ b/include/fat.h
@@ -33,7 +33,7 @@
 /* Maximum Long File Name length supported here is 128 UTF-16 code units */
 #define VFAT_MAXLEN_BYTES	256 /* Maximum LFN buffer in bytes */
 #define VFAT_MAXSEQ		9   /* Up to 9 of 13 2-byte UTF-16 entries */
-#define LINEAR_PREFETCH_SIZE	(mydata->sect_size*2) /* Prefetch buffer size */
+#define PREFETCH_BLOCKS		2
 
 #define MAX_CLUSTSIZE	65536
 #define DIRENTSPERBLOCK	(mydata->sect_size / sizeof(dir_entry))