net: stmmac: fix cbs configuration

Sending again, because forgot to include net-dev.

The QoS IP does not accept AVB capabilities to default/queue 0, this way we
guarantee 75% bandwidth for AVB. This patch assures that only queues >= 1
gets CBS confgured. Additional info was also added to stmmac.txt.

Reported-by: Niklas Cassel <niklas.cassel@axis.com>
Signed-off-by: Joao Pinto <jpinto@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>