r8169: don't pass net_device to irq coalescing sub-functions

The net_device argument is just used to get a struct rtl8169_private
pointer via netdev_priv(). Therefore pass the struct rtl8169_private
pointer directly.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>