net: stmmac: adding multiple buffers for TX

This patch adds the structure stmmac_tx_queue which contains
tx queues specific data (previously in stmmac_priv).

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