r8152: support stopping/waking tx queue

The maximum packet number which a tx aggregation buffer could contain
is the tx_qlen.

	tx_qlen = buffer size / (packet size + descriptor size).

If the tx buffer is empty and the queued packets are more than the
maximum value which is defined above, stop the tx queue. Wake the
tx queue if tx queue is stopped and the queued packets are less than
tx_qlen.

Signed-off-by: Hayes Wang <hayeswang@realtek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>