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 61c7a080
(of: Always use 'struct device.of_node' to get device node pointer)
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
CC: Sean MacLennan <smaclennan@pikatech.com>
CC: spi-devel-general@lists.sourceforge.net
CC: devicetree-discuss@lists.ozlabs.org

drivers/spi/spi_ppc4xx.c

@@ -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;