ath6kl: fix warning for using 0 as NULL

Fixes the following sparse warning:

drivers/net/wireless/ath/ath6kl/sdio.c:716:55: warning:
 Using plain integer as NULL pointer
Signed-off-by: NWei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: NKalle Valo <kvalo@qca.qualcomm.com>

drivers/net/wireless/ath/ath6kl/sdio.c

@@ -713,7 +713,7 @@ static void ath6kl_sdio_cleanup_scatter(struct ath6kl *ar)
 		 * that the packet is properly freed?
 		 */
 		if (s_req->busrequest) {
-			s_req->busrequest->scat_req = 0;
+			s_req->busrequest->scat_req = NULL;
 			ath6kl_sdio_free_bus_req(ar_sdio, s_req->busrequest);
 		}
 		kfree(s_req->virt_dma_buf);