PCI: Add pci_resource_num() helper

PCI: Add pci_resource_num() helper

A few places in PCI code, mainly in setup-bus.c, need to reverse lookup the
index of a resource in pci_dev's resource array. Create pci_resource_num()
helper to avoid repeating the pointer arithmetic trick used to calculate
the index.

Link: https://lore.kernel.org/r/20241216175632.4175-11-ilpo.jarvinen@linux.intel.com
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Tested-by: Xiaochun Lee <lixc17@lenovo.com>