PCI: Add device flag helper functions

Add helper functions to hide direct device flag operations:

    void pci_set_dev_assigned(struct pci_dev *dev);
    void pci_clear_dev_assigned(struct pci_dev *dev);
    bool pci_is_dev_assigned(struct pci_dev *dev);

Signed-off-by: Ethan Zhao <ethan.zhao@oracle.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>