r8169: replace event_slow with irq_mask

Recently the "slow event" handler was removed, therefore the member
name isn't appropriate any longer. In addition store the full mask,
including the RTL_EVENT_NAPI interrupt source bits.

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