sfc: check async completer is !NULL before calling

Add a NULL check before calling asynchronous MCDI completion functions
during device removal.

Fixes: 7014d7f6 ("sfc: allow asynchronous MCDI without completion function")
Signed-off-by: Bert Kenward <bkenward@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/sfc/mcdi.c

@@ -1156,7 +1156,8 @@ void efx_mcdi_flush_async(struct efx_nic *efx)
 	 * acquired locks in the wrong order.
 	 */
 	list_for_each_entry_safe(async, next, &mcdi->async_list, list) {
-		async->complete(efx, async->cookie, -ENETDOWN, NULL, 0);
+		if (async->complete)
+			async->complete(efx, async->cookie, -ENETDOWN, NULL, 0);
 		list_del(&async->list);
 		kfree(async);
 	}