sfxge(4): cleanup: make TLV scans quieter

Find end of segments in a more direct way that avoids an error report at
the terminator.

Submitted by: Richard Houldsworth <rhouldsworth at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D6367