net: stmmac: Restore DT backwards-compatibility

Recent changes to support multiple queues in the device tree bindings
resulted in the number of RX and TX queues to be initialized to zero for
device trees not adhering to the new bindings.

Restore backwards-compatibility with those device trees by falling back
to a single RX and TX queues each.

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