nfp: add support for link auto negotiation

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>