fs: fat: fix fat iteration

According to the FAT specification it is valid to have files with an
attribute value of 0x0. This fixes a regression where different U-Boot
versions are showing different amount of files on the same storage
device. With this change U-Boot shows the same number of files and folders
as Linux and Windows.

Fixes: 39606d46 ("fs: fat: handle deleted directory entries correctly")
Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>