PCI: Convert Downstream Port Containment driver to use devm_* functions

Use the device resource management (devm) interfaces so we don't need to
explicitly release resources on failure paths or when the driver is
removed.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Keith Busch <keith.busch@intel.com>