tcp: keep syncache flags when updating ECN info

While processing the ECN flags of an incoming packet,
incorrectly cleared all other syncache flags.

Reported by: tuexen
Reviewed By: tuexen, transport
Sponsored by: NetApp, Inc.
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D46694

(cherry picked from commit 0a05ea1f56e65ec0477d56daf5ed623087464082)