forcedeth: fix irq clearing and napi spin lock changes

This patch clears the irqstatus register with the exact same events it
has read from it. Since the read-write operation is not atomic, a new
irqstatus bit could have been set in between these operations and would
then be cleared accidentally.

Secondly, we now don't need any spin lock protection when
scheduling/completing napi poll as the isr will not execute anymore (as
we turn off all interrupts now).

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