ixgb: recalculate after how many descriptors to wake the queue

Recalculate when to wake the queue using DESC_NEEDED instead of a
static hardcoded number.

Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
diff --git a/drivers/net/ixgb/ixgb_main.c b/drivers/net/ixgb/ixgb_main.c
index fee4c5a..960b44b 100644
--- a/drivers/net/ixgb/ixgb_main.c
+++ b/drivers/net/ixgb/ixgb_main.c
@@ -1787,7 +1787,7 @@
 	if (unlikely(netif_queue_stopped(netdev))) {
 		spin_lock(&adapter->tx_lock);
 		if (netif_queue_stopped(netdev) && netif_carrier_ok(netdev) &&
-		    (IXGB_DESC_UNUSED(tx_ring) > IXGB_TX_QUEUE_WAKE))
+		    (IXGB_DESC_UNUSED(tx_ring) >= DESC_NEEDED))
 			netif_wake_queue(netdev);
 		spin_unlock(&adapter->tx_lock);
 	}