x86/PCI: Drop return value of pcibios_scan_root()

Nobody really uses the return value of pcibios_scan_root() (one place uses
it to control a printk, but the printk is not very useful).  This converts
pcibios_scan_root() to a void function.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>