fs: fat: do not mangle short filenames

Do not mangle lower or mixed case filenames which fit into the upper
case 8.3 short filename. This ensures FAT standard compatible short
filenames (SFN) to support systems without long filename (LFN) support
like boot roms (ex. SFN BOOT.BIN instead of BOOT~1.BIN for LFN
boot.bin).

Signed-off-by: Stefan Herbrechtsmeier <stefan.herbrechtsmeier@weidmueller.com>