forcedeth: add new optimization mode

A new optimization mode called Dynamic has been added. This will be mode
where interrupt moderation logic will dynamically switch between pure
throughput mode and poll based (called 'cpu') mode.

Also, for newer chipsets, the timer irq is not needed for throughput
mode. Secondly, since we are modifying the irqmask to change between
modes, msix is not supported.
Signed-off-by: Ayaz Abdulla <aabdulla@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>