of: fdt: Fix return value in __unflatten_device_tree()

Commit 78de28c ("of: fdt: add missing allocation-failure check")
would make us return NULL in a function declared to return void as of
the 4.1 kernel.

Fixes: 78de28c ("of: fdt: add missing allocation-failure check")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>

drivers/of/fdt.c

@@ -413,7 +413,7 @@ static void __unflatten_device_tree(void *blob,
 	/* Allocate memory for the expanded device tree */
 	mem = dt_alloc(size + 4, __alignof__(struct device_node));
 	if (!mem)
-		return NULL;
+		return;
 
 	memset(mem, 0, size);