sandbox: fix wget test failure after fixing wget issue

sandbox: fix wget test failure after fixing wget issue

After applying the following patch, wget test on sandbox failed[1].

  Commit: cab7867cff ("net: wget: Support retransmission a dropped packet")

Here are two reasons why the test is failed and how to fix it:

1. tcp_ack is calculated by the wrong value.
   tcp_ack needs to be calculated by the received TCP payload size.
2. wget command may have a problem that HTTP response from server
   must be divided into more than two packets.
   In this commit, HTTP response is divided into two packets.

In addition, I fixed the HTTP response returned at the correct timing.

[1] https://lore.kernel.org/u-boot/CAFLszThEbk2Jr8OZ6Hj21wPSnJjgJhaDe037RqwHvwt1KjB3_A@mail.gmail.com/

Signed-off-by: Yasuharu Shibata <yasuharu.shibata@gmail.com>
Reported-by: Simon Glass <sjg@chromium.org>