net: mvpp2: limit TSO segments and use stop/wake thresholds

Too many TSO descriptors can be required for the default queue size,
when using small MSS values for example. Prevent this by adding a
maximum number of allowed TSO segments (300). In addition set a stop and
a wake thresholds to stop the queue when there's no room for a 1 "worst
case scenario skb". Wake up the queue when the number of descriptors is
low enough.

Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>