PCI: cadence: Use FIELD_GET()

Convert open-coded variants of PCI field access into FIELD_GET() to
make the code easier to understand.

Link: https://lore.kernel.org/r/20231018113254.17616-2-ilpo.jarvinen@linux.intel.com


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