sfxge: update NVRAM partition lookup for Medford

sfxge: update NVRAM partition lookup for Medford

Prior to Medford, option ROM config was stored with one partition
per network port. Medford stores option ROM config in a single
partition (as an array of configurations, one per PF).

Update the EFXname /port to MCDI partition mapping for this.

Submitted by:   Andy Moreton <amoreton at solarflare.com>
Reviewed by:    gnn
Sponsored by:   Solarflare Communications, Inc.
MFC after:      2 days
Differential Revision: https://reviews.freebsd.org/D4885