tcp: set IP ECN header codepoint properly

TCP RACK can cache the IP header while preparing
a new TCP packet for transmission. Thus all the
IP ECN codepoint bits need to be assigned, without
assuming a clear field beforehand.

Reviewed By: tuexen, kbowling, transport
MFC after: 3 days
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D34148