tcp: clean PRR state after ECN congestion recovery.

PRR state was not properly reset on subsequent ECN CE
events. Clean up after local transmission failures too.

Reviewed by: tuexen, cc, transport
MFC after: 3 days
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D43170

(cherry picked from commit 429f14f83ae1f6357a9455d4f075522420b01b03)