powerpc/pasemi: Use of_root in pas_pci_init()

Currently in pas_pci_init() a reference to the root node is leaked due
to a missing of_node_put(). Instead just use of_root directly.

Note that converting to of_find_compatible_node(NULL, ...) would
not be entirely equivalent, because that would check the compatible
property of the root node, whereas using of_root skips checking the root
node and start the search at the first child of the root.

Reported-by: Liang He <windhl@126.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220906010313.1296714-1-mpe@ellerman.id.au