Fix AVR warnings; FAT FS needs to use off_t instead of size_t

git-svn-id: svn://svn.code.sf.net/p/nuttx/code/trunk@3728 42af7a65-404d-4744-a932-0658087f49c3

fs/romfs/fs_romfs.c

@@ -469,7 +469,7 @@ static off_t romfs_seek(FAR struct file *filep, off_t offset, int whence)
 {
   struct romfs_mountpt_s *rm;
   struct romfs_file_s    *rf;
-  ssize_t                 position;
+  off_t                   position;
   int                     ret;
 
   fvdbg("Seek to offset: %d whence: %d\n", offset, whence);