stmmac: enable rx queues

When the hardware is synthesized with multiple queues, all queues are
disabled for default. This patch adds the rx queues configuration.
This patch was successfully tested in a Synopsys QoS Reference design.

Signed-off-by: Joao Pinto <jpinto@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>