of: Use NULL for pointers

Commit 44856819 (of/fdt: Clean up casting in unflattening path)
modified unflatten_dt_node() to take a void * for the mem parameter
instead of an unsigned long. One of the call sites wasn't updated.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Grant Likely <grant.likely@linaro.org>

Conflicts:
	drivers/of/fdt.c