forcedeth: save irq events for napi processing

This patch will save the irq events in the driver's context so that the
napi routine knows which interrupts have occurred. Subsequent changes
will be moving all interrupt processing into the napi poll routine.

Signed-off-by: Ayaz Abdulla <aabdulla@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>