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

Separate because this isn't as trivial as the other FIELD_GET() changes.

See 907830b0 ("PCI: Add a REBAR size quirk for Sapphire RX 5600 XT
Pulse")

Link: https://lore.kernel.org/r/20231010204436.1000644-3-helgaas@kernel.orgSigned-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Cc: Nirmoy Das <nirmoy.das@amd.com>