sfxge: add sanity checking for EFX_OPT_MEDFORD build option to efx_check.h

Submitted by: Mark Spender <mspender at solarflare.com>
Reviewed by: gnn
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D4854