tg3: update rx_jumbo_pending ring param only when jumbo frames are enabled

The patch fixes a problem with dropped jumbo frames after usage of
'ethtool -G ... rx'.

Scenario:
1. ip link set eth0 up
2. ethtool -G eth0 rx N # <- This zeroes rx-jumbo
3. ip link set mtu 9000 dev eth0

The ethtool command set rx_jumbo_pending to zero so any received jumbo
packets are dropped and you need to use 'ethtool -G eth0 rx-jumbo N'
to workaround the issue.
The patch changes the logic so rx_jumbo_pending value is changed only if
jumbo frames are enabled (MTU > 1500).

Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Acked-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/broadcom/tg3.c

@@ -12286,7 +12286,9 @@ static int tg3_set_ringparam(struct net_device *dev, struct ethtool_ringparam *e
 	if (tg3_flag(tp, MAX_RXPEND_64) &&
 	    tp->rx_pending > 63)
 		tp->rx_pending = 63;
-	tp->rx_jumbo_pending = ering->rx_jumbo_pending;
+
+	if (tg3_flag(tp, JUMBO_RING_ENABLE))
+		tp->rx_jumbo_pending = ering->rx_jumbo_pending;
 
 	for (i = 0; i < tp->irq_max; i++)
 		tp->napi[i].tx_pending = ering->tx_pending;