pcie: Fixed return nothing when get PCIeDeviceId

GET /redfish/v1/Systems/system/PCIeDevices/<str> nothing will be
returned if their 'generationInUse' is Unknown.
The original intention is that if it is Unknown, there is no need to
display '"PCIeType"'. But the others still have to show.

After fix:
PCIeType is not Unknown:
GET http://${bmc}/redfish/v1/Systems/system/PCIeDevices/<str>
{
  "@odata.id": "/redfish/v1/Systems/system/PCIeDevices/<str>.",
  "@odata.type": "#PCIeDevice.v1_4_0.PCIeDevice",
  "DeviceType": "SingleFunction",
  "Id": "<str>",
  "Manufacturer": "Intel Corporation",
  "Name": "PCIe Device",
  "PCIeFunctions": {
"@odata.id":
"/redfish/v1/Systems/system/PCIeDevices/<str>/PCIeFunctions"
  },
  "PCIeInterface": {
    "PCIeType": "Gen1"
  }
}

PCIeType is Unknown:
GET http://${bmc}/redfish/v1/Systems/system/PCIeDevices/<str>
{
  "@odata.id": "/redfish/v1/Systems/system/PCIeDevices/<str>",
  "@odata.type": "#PCIeDevice.v1_4_0.PCIeDevice",
  "DeviceType": "SingleFunction",
  "Id": "<str>",
  "Manufacturer": "Intel Corporation",
  "Name": "PCIe Device",
  "PCIeFunctions": {
    "@odata.id":
    "/redfish/v1/Systems/system/PCIeDevices/<str>/PCIeFunctions"
  }
}

Signed-off-by: Tony Lee <tony.lee@quantatw.com>
Change-Id: I85a97e0370a5789e4526964272381a557ce942a0