crypto: hisilicon - Fix using plain integer as NULL pointer

This patch fix sparse warning:
zip_crypto.c:425:26: warning: Using plain integer as NULL pointer

Replaces assignment of 0 to pointer with NULL assignment.
Signed-off-by: Shukun Tan <tanshukun1@huawei.com>
Signed-off-by: Zhou Wang <wangzhou1@hisilicon.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

drivers/crypto/hisilicon/zip/zip_crypto.c

@@ -42,7 +42,7 @@ enum hisi_zip_alg_type {
 
 #define TO_HEAD(req_type)						\
 	(((req_type) == HZIP_ALG_TYPE_ZLIB) ? zlib_head :		\
-	 ((req_type) == HZIP_ALG_TYPE_GZIP) ? gzip_head : 0)		\
+	 ((req_type) == HZIP_ALG_TYPE_GZIP) ? gzip_head : NULL)		\
 
 struct hisi_zip_req {
 	struct acomp_req *req;