Follow-up to r1466918: toggle bandwidth threshold condition.

* subversion/svnserve/svnserve.c
  (main): optimize when it's *at least* 1Gb/s

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1466925 13f79535-47bb-0310-9956-ffa450edef68