PCI: tegra: Clear CLKREQ# enable on port disable

When a root port is disabled, disable the CLKREQ# signal if available.

Tested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>