PCI: Disable Gen2 for Tegra20 and Tegra30

Tegra20 and Tegra30 do not support gen2 PCIe, so correct the
register setting to disable it.

Signed-off-by: Eric Brower <ebrower@nvidia.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Thierry Reding <treding@nvidia.com>