ixgbe: Add support for 1Gbit DAC links

This is a relatively well known trick for the X520 (82599), can be
useful for testing and lab settings. It's not an official standard or
particularly common but ubiquitous Broadcom switch ASICs deal with it.

We'll call it 1000Base-KX because it's SerDes on the passive cable and
I don't think it's worth adding another media type for this.

Reviewed by: emaste
MFC after: 1 week
Sponsored by: BBOX.io
Differential Revision: https://reviews.freebsd.org/D47352