fs/fat: Fix unaligned __u16 reads for FAT12 access

Doing unaligned reads is not supported on all architectures, use
byte sized reads of the little endian buffer.
Rename off16 to off8, as it reflects the buffer offset in byte
granularity (offset is in entry, i.e. 12 bit, granularity).
Fix a regression introduced in 8d48c92b

Reported-by: Oleksandr Tymoshenko <gonzo@bluezbox.com>
Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Tested-by: Oleksandr Tymoshenko <gonzo@bluezbox.com>