net: tcp2: Fix TCP connection from Windows 10

Windows 10 sends ECN-Echo and Congestion Window Reduced (CWR) flags
together with SYN flag in the connection establishment but the code
did not ignore these flags and send just SYN back (instead of SYN|ACK).
This caused the connection establishement in application level to
fail as the application was never notified about it.

Fixes #29258

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>