net: tcp: Set the right th_win value in tester_prepare_tcp_pkt

The 16bit th_win in the TCP header was not set via htons(). It
will bring issues of wrong th_win value in the peer end. For
example, the 1280 (0x0500) TCP window size would be taken as
5 (0x0005) by the peer end.

Signed-off-by: Shrek Wang <inet_eman@outlook.com>