PCI: xilinx-nwl: Add missing of_node_put()

The call to of_get_next_child() returns a node pointer with
refcount incremented thus it must be explicitly decremented
here after the last usage.

Fixes: ab597d35 ("PCI: xilinx-nwl: Add support for Xilinx NWL PCIe Host Controller")
Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
[lorenzo.pieralisi@arm.com: updated commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>