netdev: octeon_mgmt: Improve ethtool_ops.

Correctly show no link when the interface is down, and return
-EOPNOTSUPP for things that don't work.  This quiets the ethtool
program when run on down interfaces.

Signed-off-by: David Daney <david.daney@cavium.com>
Acked-by: David S. Miller <davem@davemloft.net>