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

[arybchik]

Summary

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.

Test Plan

amd64 tinderbox kernel builds tested
driver loaded/unloaded, interfaces up/down, traffic works fine