net/mlx4_en: cq->irq_desc wasn't set in legacy EQ's

Fix a regression introduced by commit 35f6f453 ("net/mlx4_en: Don't use
irq_affinity_notifier to track changes in IRQ affinity map").
When core is started in legacy EQ's (number of IRQ's < rx rings), cq->irq_desc
was NULL.  This caused a kernel crash under heavy traffic - when having more
than rx NAPI budget completions.
Fixed to have it set for both EQ modes.
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mellanox/mlx4/en_cq.c

@@ -129,14 +129,15 @@ int mlx4_en_activate_cq(struct mlx4_en_priv *priv, struct mlx4_en_cq *cq,
 						  name);
 				}
 
-				cq->irq_desc =
-					irq_to_desc(mlx4_eq_get_irq(mdev->dev,
-								    cq->vector));
 			}
 		} else {
 			cq->vector = (cq->ring + 1 + priv->port) %
 				mdev->dev->caps.num_comp_vectors;
 		}
+
+		cq->irq_desc =
+			irq_to_desc(mlx4_eq_get_irq(mdev->dev,
+						    cq->vector));
 	} else {
 		/* For TX we use the same irq per
 		ring we assigned for the RX    */