forcedeth: limit tx to 16

This is a critical patch which adds a workaround for a HW bug. The patch
will limit the number of outstanding tx packets to 16. Otherwise, the HW
could send out packets with bad checksums.

The driver will still setup the tx packets into the ring, however, will
only set the Valid bit on 16 packets at a time.
Signed-off-by: Ayaz Abdulla <aabdulla@nvidia.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>