fs: fat_write: fix the incorrect last cluster checking

In fat_write.c, the last clust condition check is incorrect:

  if ((curclust >= 0xffffff8) || (curclust >= 0xfff8)) {
  	... ...
  }

For example, in FAT32 if curclust is 0x11000. It is a valid clust.
But on above condition check, it will be think as a last clust.

So the correct last clust check should be:
  in fat32, curclust >= 0xffffff8
  in fat16, curclust >= 0xfff8
  in fat12, curclust >= 0xff8

This patch correct the last clust check.

Signed-off-by: Josh Wu <josh.wu@atmel.com>

fs/fat/fat_write.c

@@ -882,7 +882,7 @@ static dir_entry *find_directory_entry(fsdata *mydata, int startsect,
 		}
 
 		curclust = get_fatent_value(mydata, dir_curclust);
-		if ((curclust >= 0xffffff8) || (curclust >= 0xfff8)) {
+		if (IS_LAST_CLUST(curclust, mydata->fatsize)) {
 			empty_dentptr = dentptr;
 			return NULL;
 		}

include/fat.h

@@ -84,6 +84,9 @@
 #define START(dent)	(FAT2CPU16((dent)->start) \
 			+ (mydata->fatsize != 32 ? 0 : \
 			  (FAT2CPU16((dent)->starthi) << 16)))
+#define IS_LAST_CLUST(x, fatsize) ((x) >= ((fatsize) != 32 ? \
+					((fatsize) != 16 ? 0xff8 : 0xfff8) : \
+					0xffffff8))
 #define CHECK_CLUST(x, fatsize) ((x) <= 1 || \
 				(x) >= ((fatsize) != 32 ? \
 					((fatsize) != 16 ? 0xff0 : 0xfff0) : \