x86/pci: Use cached pci_dev->pcie_cap to simplify code

The PCI core caches the PCIe Capability offset in pci_dev->pcie_cap, so
use that instead of pci_find_capability().  Use pci_bus_set_ops() when
replacing the device pci_ops.  And use #defines instead of numeric
constants.

[bhelgaas: changelog, also use PCI_EXP_LNKCTL_ASPMC]
Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>