Introduce pci_is_pcie() which returns true if the specified PCI device
is PCI Express capable, false otherwise.
The purpose of pci_is_pcie() is removing 'is_pcie' flag in the struct
pci_dev, which is not needed because we can check it using 'pcie_cap'
field. To remove 'is_pcie', we need to update user of 'is_pcie' to use
pci_is_pcie() instead first.