Fix TFTP OACK code for short packets.

The old code had a loop limit overflow bug which caused a semi-
infinite loop for small packets, because in "i<len-8", "i" was signed,
but "len" was unsigned, and "len-8" became a huge number for small
values of "len".

This is a workaround which replaces broken commit 8f1bc284.

Signed-off-by: Wolfgang Denk <wd@denx.de>