PCI: add Ralink RT2800 broken INTx masking quirk

Passes pci_intx_mask_supported test but continues to send interrupts
as discovered through VFIO-based device assignment.

http://www.spinics.net/lists/kvm/msg73738.html

[bhelgaas: use HEADER, not FINAL, which is currently broken for hotplug]
Tested-by: Andreas Hartmann <andihartmann@01019freenet.de>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>