forcedeth: remove isr processing loop

This patch is only a subset of changes so that it is easier to see the
modifications. This patch removes the isr 'for' loop and shifts all the
logic to account for new tab spacing.

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