include/linux/of.h: make for_each_child_of_node() reference its args when CONFIG_OF=n

Make for_each_child_of_node() reference its args when CONFIG_OF=n to
avoid warnings like:

	drivers/leds/leds-pwm.c:88:22: warning: unused variable 'node' [-Wunused-variable]
	  struct device_node *node = pdev->dev.of_node;
			      ^

Signed-off-by: David Howells <dhowells@redhat.com>
Cc: Grant Likely <grant.likely@linaro.org>
Cc: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>