PCI: Cache the VF device ID in the SR-IOV structure

Cache the VF device ID in the SR-IOV structure and use it instead of
reading it over and over from the PF config space capability.

Signed-off-by: Filippo Sironi <sironi@amazon.de>
[bhelgaas: rename to "vf_device" to match pci_dev->device]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>