tcp: make ghost ACK handling more conformant

The latest version of draft-ietf-tcpm-tcp-ghost-acks changed
a condition. This should make no substantial difference, but makei
it compliant to the latest version of the specification.

Reviewed by: rscheff, Peter Lei
MFC after: 3 days
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D58411