PCI: pciehp: Use PCI_EXP_SLTCAP_PSN define

Use PCI_EXP_SLTCAP_PSN to make it easier to find code that uses the
Physical Slot Number field in the PCIe Slot Capabilities register.

No functional change.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>