Support for FEC encoding control

This is a request to backport the following upstream commit which is present in v4.15.

commit 8db75d1e4001ac4cdfc73d5bedd0ec6d58a3d617
Author: Dustin Byford <email address hidden>
Date: Mon Dec 18 14:57:41 2017 -0800

    ethtool: Support for FEC encoding control

    As FEC settings and different FEC modes are mandatory
    and configurable across various interfaces of 25G/50G/100G/40G,
    the lack of FEC encoding control and reporting today is a source
    for interoperability issues for many vendors

    set-fec/show-fec option(s) are designed to provide control and report
    the FEC encoding on the link.

    $ethtool --set-fec swp1 encoding [off | RS | BaseR | auto]

    Encoding: Types of encoding
    Off : Turning off FEC
    RS : Force RS-FEC encoding
    BaseR : Force BaseR encoding
    Auto : Default FEC settings for drivers, and would represent
              asking the hardware to essentially go into a best effort mode.

    Here are a few examples of what we would expect if encoding=auto:
    - if autoneg is on, we are expecting FEC to be negotiated as on or off
      as long as protocol supports it
    - if the hardware is capable of detecting the FEC encoding on it's
      receiver it will reconfigure its encoder to match
    - in absence of the above, the configuration would be set to IEEE
      defaults.

    >From our understanding, this is essentially what most hardware/driver
    combinations are doing today in the absence of a way for users to
    control the behavior.

    $ethtool --show-fec swp1
    FEC parameters for swp1:
    FEC encodings: RS

    ethtool devname output:
    $ethtool swp1
    Settings for swp1:
    root@hpe-7712-03:~# ethtool swp18
    Settings for swp18:
        Supported ports: [ FIBRE ]
        Supported link modes: 40000baseCR4/Full
                                40000baseSR4/Full
                                40000baseLR4/Full
                                100000baseSR4/Full
                                100000baseCR4/Full
                                100000baseLR4_ER4/Full
        Supported pause frame use: No
        Supports auto-negotiation: Yes
        Supported FEC modes: [RS | BaseR | None | Not reported]
        Advertised link modes: Not reported
        Advertised pause frame use: No
        Advertised auto-negotiation: No
        Advertised FEC modes: [RS | BaseR | None | Not reported]
        Speed: 100000Mb/s
        Duplex: Full
        Port: FIBRE
        PHYAD: 106
        Transceiver: internal
        Auto-negotiation: off
        Link detected: yes

    Signed-off-by: Vidya Sagar Ravipati <email address hidden>
    Signed-off-by: Dustin Byford <email address hidden>
    [code style + man page edits + commit message update]
    Signed-off-by: Dirk van der Merwe <email address hidden>
    Signed-off-by: John W. Linville <email address hidden>