PCI/DPC: Add and use DPC Status register field definitions

Add definitions for DPC Status register fields and use them in the code.
No functional change intended.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Sinan Kaya <okaya@codeaurora.org>