sfxge(4): report correct partition write chunk size

If the firmware reports a non-zero write chunk size then nvram writes
may fail if a different granularity is used (e.g. for MUM firmware on
Sorrento).

Submitted by: Andy Moreton <amoreton at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18091