[POWERPC] arch/powerpc/sysdev/cpm2_pic.c of_node_get cleanup

Remove redundant argument check for of_node_get().
It's ok to remove 'node' check because in real life cpm2_pic_init()
never gets called with node == NULL.

Signed-off-by: Mariusz Kozlowski <m.kozlowski@tuxland.pl>
Signed-off-by: Paul Mackerras <paulus@samba.org>