PCI/DPC: Use defines with DPC reason fields

Add new defines for DPC reason fields and use them instead of literals.

Link: https://lore.kernel.org/r/20231018113254.17616-7-ilpo.jarvinen@linux.intel.com


Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
[bhelgaas: shorten comments]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>