forcedeth: performance changes

This patch modifies the throughput mode poll settings to reduce the
number of interrupts. This is only used by older hardware that need a
timer irq in throughput mode.

Secondly, this patch increases the default rx ring from 128 to 512. This
drastically improves bandwidth utilization for small packets sizes i.e
512 bytes.

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