Report the auto negotiation capability if it's supported
in management firmware, and advertise it if it's enabled.
Changing port speed is not allowed when autoneg is enabled.

The ethtool <intf> command displays the auto-neg capability:

  # ethtool enp1s0np0
  Settings for enp1s0np0:
          Supported ports: [ FIBRE ]
          Supported link modes:   Not reported
          Supported pause frame use: Symmetric
          Supports auto-negotiation: Yes
          Supported FEC modes: None        RS      BASER
          Advertised link modes:  Not reported
          Advertised pause frame use: Symmetric
          Advertised auto-negotiation: Yes
          Advertised FEC modes: None       RS      BASER
          Speed: 25000Mb/s
          Duplex: Full
          Auto-negotiation: on
          Port: FIBRE
          PHYAD: 0
          Transceiver: internal
          Link detected: yes
Signed-off-by: Yinjun Zhang <yinjun.zhang@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>