forcedeth: new backoff implementation

This patch adds support for a new backoff algorithm for half duplex supported
in newer hardware.  The old method is will be designated as legacy mode.

Re-seeding random values for the backoff algorithms are performed when a
transmit has failed due to a maximum retry count (1 to 15, where max is
considered the wraparound case of 0).

Signed-off-by: Ayaz Abdulla <aabdulla@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>