forcedeth: tx pause fix

This patch fixes the tx pause enable watermark flags. The new values
where determined to be optimal during testing.

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