sunvnet: rename vnet_port_alloc_tx_bufs and move after version negotiation

This patch changes the name of vnet_port_alloc_tx_bufs to
vnet_port_alloc_tx_ring, since there are no buffer allocations after
transmit zero copy support was added. This patch also moves the ring
allocation to after VIO version negotiation to allow for
different-sized descriptors in later VIO versions.

Signed-off-by: David L Stevens <david.stevens@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>