forcedeth: tx pause watermarks

New chipsets introduced variant Rx FIFO sizes that need to be taken into
account when setting up the tx pause watermarks. This patch introduces
the new device feature flags based on a version and implements the new
watermarks.

Signed-off-by: Ayaz Abdulla <aabdulla@nvidia.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>