fat: fatwrite: fix the command for FAT12

The u-boot command fatwrite empties FAT clusters from the beginning
till the end of the file.
Specifically for FAT12 it fails to detect the end of the file and goes
beyond the file bounds thus corrupting the file system.

Additionally, FAT entry chaining-up into a file is not implemented
for FAT12.

The users normally workaround this by re-formatting the partition as
FAT16/FAT32, like here:
https://github.com/FEDEVEL/openrex-uboot-v2015.10/issues/1



The patch fixes the bounds of a file and FAT12 entries chaining into
a file, including EOF markup.

Signed-off-by: Philipp Skadorov <philipp.skadorov@savoirfairelinux.com>