Do not blindly announce 1000baseT half-duplex capability in
autonegotiation.  Some controllers like cgem(4) do not support
half-duplex at gigabit speeds.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271073 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f