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>