neta: split fixed and in-band link status configuration

Fixed-link mode requires different handling than the in-band
managed connection. Update interrupt, link-up/down and
autonegotiation settings for the former.

Reviewed by: mw
MFC after: 1 week
Obtained from: Stormshield
Differential Revision: https://reviews.freebsd.org/D34394

(cherry picked from commit 231237bbb0e803f9047c235cd6b5878f70e6c5eb)