net: bcmgenet: adjust the call to alloc_etherdev_mqs()

In preparation for supporting multiple Rx queues, adjust the call to
alloc_etherdev_mqs() to allow max GENET_MAX_MQ_CNT + 1 Rx queues.

The actual number of Rx queues in use is correctly adjusted with:
netif_set_real_num_rx_queues(priv->dev, priv->hw_params->rx_queues + 1);

Signed-off-by: Petri Gynther <pgynther@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>