powerpc/pseries: delete unneeded test before of_node_put

Of_node_put supports NULL as its argument, so the initial test is not
necessary.

Suggested by Uwe Kleine-König.

The semantic patch that fixes this problem is as follows:
(http://coccinelle.lip6.fr/)

// <smpl>
@@
expression e;
@@

-if (e)
   of_node_put(e);
// </smpl>
Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/platforms/pseries/iommu.c

@@ -574,7 +574,6 @@ static void pci_dma_bus_setup_pSeries(struct pci_bus *bus)
 	while (isa_dn && isa_dn != dn)
 		isa_dn = isa_dn->parent;
 
-	if (isa_dn_orig)
 	of_node_put(isa_dn_orig);
 
 	/* Count number of direct PCI children of the PHB. */