fbnic: Replace link_mode with AUI

The way we were using "link_mode" was really more to describe the interface
between the attachment unit interface(s) we were using on the device.
Specifically the AUI is describing the modulation and the number of lanes
we are using. So we can simplify this by replacing link_mode with aui.

Signed-off-by: Alexander Duyck <alexanderduyck@fb.com>
Link: https://patch.msgid.link/175028446219.625704.8050098629750896117.stgit@ahduyck-xeon-server.home.arpa


Signed-off-by: Paolo Abeni <pabeni@redhat.com>