PCI/DPC: Use FIELD_GET()

Use FIELD_GET() to remove dependencies on the field position, i.e., the
shift value. No functional change intended.

Link: https://lore.kernel.org/r/20231018113254.17616-5-ilpo.jarvinen@linux.intel.com


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