r8169: handle all interrupt events in the hard irq handler

Having a separate "slow event" handler isn't needed because all
interrupt events trigger asynchronous activity. And in case of SYSErr
we have bigger problems than performance anyway.
This patch also allows to get rid of acking interrupt events in the
NAPI poll callback.

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