Modify the transmit packet queuing strategy a bit to be a little less

Modify the transmit packet queuing strategy a bit to be a little less
agressive. With the old code, if a descriptor chain was already on its
way to the chip, xl_start() would try to splice new chains onto the end
of the current chain by stopping the transmitter, modifying the tail
pointer of the current chain to point to the head of the new chain, then
restart the transmitter. The manual says you're allowed to do this and
it works, but I'm not too keen on it anymore.

The new code waits until the eixsting chain has been sent and then
queues the next waiting chain in the 'transmit ok' handler.

Performance still looks good one way or the other.