FAT: Fix file contents listed as directory

With:
fatls mmc 0 /dir/file
dir: regular directory
file: regular file

The previous code read the contents of file as if it were directory entries to
list. This patch refuses to list file contents as if it were a folder.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Cc: Wolfgang Denk <wd@denx.de>