sfxge(4): report verify result from RW finish callback

This makes the verify result visible to efx_nvram_rw_finish(), which
can be extended to report it in a later patch.

Submitted by: Andy Moreton <amoreton at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18086