net: stmmac: Use AVB mode by default

Prior to the recent multi-queue changes the driver would configure the
queues to use the AVB mode, but the mode then got switched to DCB. The
hardware still works fine in DCB mode, but my testing capabilities are
limited, so it's safer to revert to the prior setting anyway.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Acked-By: Joao Pinto <jpinto@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>