igc: Change default duplex setting

During media selection through ifconfig one might not specify
the duplex setting through the mediaopt flag.
In that case the igc driver would default to full-duplex.
The problem with this approach is that e1000(em/igb) driver
defaults to half-duplex.
Because of that if one connects both NICs and sets media to
e.g. 100baseTX on both of them no link will be found.
Fix that by matching igc behaviour with what e1000 does.

Reviewed by: grehan
Approved by: mw(mentor)
Obtained from: Semihalf
Sponsored by: Stormshield
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D35673