PCI/PTM: Use FIELD_GET()

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

Link: https://lore.kernel.org/r/20231010204436.1000644-9-helgaas@kernel.org


Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>