PCI: tegra: Remove tegra_pcie_scan_bus()

After b97ea289

 ("PCI: Assign resources before drivers claim devices
(pci_scan_root_bus())"), pci_scan_root_bus() no longer adds the devices, so
it is equivalent to tegra_pcie_scan_bus().

Remove tegra_pcie_scan_bus() (the hw.scan method), so we use the generic
pci_scan_root_bus() path.

[bhelgaas: changelog]
Tested-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Thierry Reding <treding@nvidia.com>