sfxge(4): restructure TLV buffer validation

Move TLV buffer validation into ef10-specific function and add accessor
function which also converts the partition ID to the internal
representation.

Submitted by: Richard Houldsworth <rhouldsworth at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D6263