ixgbe: Add support for 1Gbit DAC links

[kbowling]

Summary

This is a relatively well known trick for the X520 (82599), can be useful for testing and lab settings.

[emaste]

LGTM

sys/dev/ixgbe/ixgbe_82599.c

446–447

was this case picked up elsewhere before?

[emaste]

Would be nice to make a note of this in the man page as well (although there might be a lot of other info to backfill first, if going that way).

[kbowling]

sys/dev/ixgbe/ixgbe_82599.c

446–447

it would fall under the eeprom control below which would yield a 10g link unless someone alters their eeprom to force 1g.