net: tftp: Fix load_block offset calculation

When load the last block, the "len" might not be a block size. This cause
loading the incorrect last block data.

The fix change "len" to tftp_block_size and minus one tftp_block_size
for offset calculation.

Use same offset calculation formula as in store_block().

Signed-off-by: Ley Foon Tan <ley.foon.tan@intel.com>
Reviewed-By: Ramon Fried <rfried.dev@gmail.com>