tcp: fix conversion of rttvar

A wrong variable and wrong scaling factors were used.

Reported by: Coverity Scan
CID: 1508689
Reviewed by: rscheff
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D44612

(cherry picked from commit e0bd180130b8c95f568483d0df6abff00d7d2153)