ethernet: Expose FullDuplex property on Redfish

Read FullDuplex from D-Bus and expose it in the Redfish
EthernetInterface GET response.

Tested:
Get /redfish/v1/Managers/bmc/EthernetInterfaces/eth0
{
  "@odata.id": "/redfish/v1/Managers/bmc/EthernetInterfaces/eth0",
  "@odata.type": "#EthernetInterface.v1_9_0.EthernetInterface",
  "DHCPv4": {
    "DHCPEnabled": true,
    "UseDNSServers": true,
    "UseDomainName": true,
    "UseNTPServers": true
  },
  "DHCPv6": {
    "OperatingMode": "Enabled",
    "UseDNSServers": true,
    "UseDomainName": true,
    "UseNTPServers": true
  },
  "Description": "Management Network Interface",
  "EthernetInterfaceType": "Physical",
  "FQDN": "evb-ast2600",
  "FullDuplex": true,
  "HostName": "evb-ast2600",
  "IPv4Addresses": [
    {
      "Address": "x.x.x.x",
      "AddressOrigin": "DHCP",
      "Gateway": "x.x.x.x",
      "SubnetMask": "255.255.255.0"
    }
  ],
  "IPv4StaticAddresses": [],
  "IPv6AddressPolicyTable": [],
  "IPv6Addresses": [
    {
      "Address": "fe80::xxxx:xxxx:xxxx:xxxx",
      "AddressOrigin": "LinkLocal",
      "PrefixLength": 64
    }
  ],
  "IPv6DefaultGateway": "0:0:0:0:0:0:0:0",
  "IPv6StaticAddresses": [],
  "IPv6StaticDefaultGateways": [],
  "Id": "eth0",
  "InterfaceEnabled": true,
  "LinkStatus": "LinkUp",
  "MACAddress": "xx:xx:xx:xx:xx:xx",
  "MTUSize": 1500,
  "Name": "Manager Ethernet Interface",
  "NameServers": [
    "x.x.x.x"
  ],
  "SpeedMbps": 1000,
  "StatelessAddressAutoConfig": {
    "IPv6AutoConfigEnabled": true
  },
  "StaticNameServers": [],
  "Status": {
    "State": "Enabled"
}

 - Redfish Service Validator v3.1.6 run against
   tree: /redfish/v1/Managers/bmc/EthernetInterfaces
   PASS: 199, WARN: 0, FAIL: 0, NOT TESTED: 98

Change-Id: I3892a75b6a023fcd435f06074011e5a199d11c08
Signed-off-by: Rashid MP <rashidmp@ami.com>