r8169: enable GRO software interrupt coalescing per default

There are reports about r8169 not reaching full line speed on certain
systems (e.g. SBC's) with a 2.5Gbps link.
There was a time when hardware interrupt coalescing was enabled per
default, but this was changed due to ASPM-related issues on few systems.
So let's use software interrupt coalescing instead and enable it
using new function netdev_sw_irq_coalesce_default_on().

Even with these conservative settings interrupt load on my 1Gbps test
system reduced significantly.
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/realtek/r8169_main.c

@@ -5283,6 +5283,8 @@ static int rtl_init_one(struct pci_dev *pdev, const struct pci_device_id *ent)
 	dev->hw_features |= NETIF_F_RXALL;
 	dev->hw_features |= NETIF_F_RXFCS;
 
+	netdev_sw_irq_coalesce_default_on(dev);
+
 	/* configure chip for default features */
 	rtl8169_set_features(dev, dev->features);