staging: gasket: use NULL instead of 0 for null pointer

Fixes sparse warning: Using plain integer as NULL pointer
Signed-off-by: Dmitriy Cherkasov <dmitriy@oss-tech.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/gasket/gasket_page_table.c

@@ -1605,7 +1605,7 @@ int gasket_alloc_coherent_memory(struct gasket_dev *gasket_dev, u64 size,
 
 	if (gasket_dev->page_table[index]->coherent_pages) {
 		kfree(gasket_dev->page_table[index]->coherent_pages);
-		gasket_dev->page_table[index]->coherent_pages = 0;
+		gasket_dev->page_table[index]->coherent_pages = NULL;
 	}
 	gasket_dev->page_table[index]->num_coherent_pages = 0;
 	return -ENOMEM;