sfxge(4): make verified update result available from ef10_nvram_partn_unlock()

Manftest needs to know exactly what went wrong with the verified update
so that failing boards can be correctly diagnosed.

Submitted by: Tom Millington <tmillington at solarflare.com>
Reviewed by: gnn
Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D8969