Use PCI_EXP_SLTCAP_PSN mask to make it easier to find where the
Physical Slot Number is used.

The Physical Slot Number is bits 31:19 of the Slot Capabilities Register,
and slot_cap is a u32, so the mask is technically unnecessary, but it's
helpful for human readers.
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>