1 Set or clear OACTIVE as transmit buffers are depleted or
replenished, respectively. Do not use 802.11 acknowledgements
as a criteria for clearing OACTIVE.
2 Let each transmit queue count down to timeout independently,
and get rid of the shared countdown (sc_tx_timer). When
we add a packet to a transmit queue, restart the queue's
countdown. Stop a transmit queue's countdown when the
queue empties.
