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