niu: Fix error handling

platform_device_register_simple() returns ERR_PTR(), not NULL, if an error 
occurs.

Found by smatch (http://repo.or.cz/w/smatch.git).  Compile tested.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/niu.c b/drivers/net/niu.c
index 73cac6c..2cd6109 100644
--- a/drivers/net/niu.c
+++ b/drivers/net/niu.c
@@ -9542,7 +9542,7 @@
 
 	plat_dev = platform_device_register_simple("niu", niu_parent_index,
 						   NULL, 0);
-	if (!plat_dev)
+	if (IS_ERR(plat_dev))
 		return NULL;
 
 	for (i = 0; attr_name(niu_parent_attributes[i]); i++) {