net: tftp: Make sure timeout will not effect wrap offset

When the block 0 store to the memory of client and timeout at this
moment. Because of no ACK packet, the server will send block 0 again,
if this client reconnect to the server at this time,
TftpBlockWrapOffset will become larger than it should be.

Signed-off-by: Rockly <rocklygnome@gmail.com>
Patch: 264417