PCI/MSI: Use FIELD_GET/PREP()

Instead of custom masking and shifting, use FIELD_GET/PREP() with
register fields.

Link: https://lore.kernel.org/r/20231018113254.17616-8-ilpo.jarvinen@linux.intel.com


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