r8169: add support for pause ethtool ops

This adds support for the [g|s]et_pauseparam ethtool ops. It considers
that the chip doesn't support pause frame use in jumbo mode.

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