netdev: octeon_mgmt: Improve ethtool_ops.

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>
Cc: linux-mips@linux-mips.org
Cc: David S. Miller <davem@davemloft.net>
Cc: netdev@vger.kernel.org
Cc: linux-kernel@vger.kernel.org,
Patchwork: https://patchwork.linux-mips.org/patch/4284/
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>