sfxge(4): treat EFX_LINK_UNKOWN as link down
It is safer to consider EFX_LINK_UNKNOWN as link down.
link_mode is set to EFX_LINK_UNKNOWN on port stop and fini.
Reviewed by: philip
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D9060