stmmac: use netif_set_real_num_{rx,tx}_queues

A driver must not access the two fields directly but should instead use
the helper functions to set the values and keep a consistent internal
state:

ethernet/stmicro/stmmac/stmmac_main.c: In function 'stmmac_dvr_probe':
ethernet/stmicro/stmmac/stmmac_main.c:4083:8: error: 'struct net_device' has no member named 'real_num_rx_queues'; did you mean 'real_num_tx_queues'?

Fixes: a8f5102a ("net: stmmac: TX and RX queue priority configuration")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>