gianfar: Use netif_set_real_num_rx_queues()

Do not set num_tx_queues or real_num_tx_queues, since
alloc_etherdev_mq() does that.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>