fs/fat: fix case for FAT shortnames

Noticed when comparing our output to linux.  There are some lcase bits
which control whether filename and/or extension should be downcase'd.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Łukasz Majewski <lukma@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>