PCI: Drop references acquired by of_parse_phandle()

of_parse_phandle() returns a device_node pointer with the refcount
incremented.  We should dispose of this reference when we're finished.

Drop the reference acquired by of_parse_phandle().

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>