void mc32_load_tx_ring (struct net_device * dev);
This sets up the host transmit data-structures.
First, we obtain from the card it's current postion in the tx ring, so that we will know where to begin transmitting packets.
Then, we read the 'next' pointers from the on-card tx ring into our tx_ring array to reduce slow shared-mem reads. Finally, we intitalise the tx house keeping variables.