igc: Change default duplex setting

[kd]

Summary

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.