PCI: Use designated initialization in PCI_VDEVICE

By using designated initialization in PCI_VDEVICE, like other similar
macros, many "missing initializer" warnings that appear when compiling with
W=2 can be silenced.

Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>