iommu/amd: Remove local PCI_BUS() define and use PCI_BUS_NUM() from PCI

Change to remove local PCI_BUS() define and use the new PCI_BUS_NUM()
interface from PCI.

Signed-off-by: Shuah Khan <shuah.khan@hp.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Joerg Roedel <joro@8bytes.org>