of/spi: Fix build failure on spi_ppc4xx.c

This patch fixes a build error caused by the OF device_node pointer being moved into struct device.

Fixes bug introduced by commit 61c7a080a5a061c976988fd4b844dfb468dda255 (of: Always use 'struct device.of_node' to get device node pointer)

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I've applied this patch to my merge-devicetree branch

 drivers/spi/spi_ppc4xx.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

index 19c0b3b..d53466a 100644
@@ -397,7 +397,7 @@ static int __init spi_ppc4xx_of_probe(struct of_device *op,
 	struct spi_master *master;
 	struct spi_bitbang *bbp;
 	struct resource resource;
-	struct device_node *np = op->node;
+	struct device_node *np = op->dev.of_node;
 	struct device *dev = &op->dev;
 	struct device_node *opbnp;
 	int ret;