PCI: Add ALIGN_DOWN_IF_NONZERO() helper

PCI: Add ALIGN_DOWN_IF_NONZERO() helper

pci_bus_distribute_available_resources() performs alignment in case of
non-zero alignment requirement on three occasions.

Add ALIGN_DOWN_IF_NONZERO() helper to avoid coding the non-zero check three
times.

Suggested-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20240614100606.15830-5-ilpo.jarvinen@linux.intel.com
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>