sfc: Fix check for failure of MC_CMD_FLUSH_RX_QUEUES

efx_mcdi_rpc_start() returns a negative value on error or zero on
success.  However one caller that can't properly handle failure then
does WARN_ON(rc > 0).  Change it to WARN_ON(rc < 0).

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
diff --git a/drivers/net/ethernet/sfc/mcdi.c b/drivers/net/ethernet/sfc/mcdi.c
index aea43cb..e1967fb 100644
--- a/drivers/net/ethernet/sfc/mcdi.c
+++ b/drivers/net/ethernet/sfc/mcdi.c
@@ -1216,7 +1216,7 @@
 
 	rc = efx_mcdi_rpc(efx, MC_CMD_FLUSH_RX_QUEUES, (u8 *)qid,
 			  count * sizeof(*qid), NULL, 0, NULL);
-	WARN_ON(rc > 0);
+	WARN_ON(rc < 0);
 
 	kfree(qid);