net: stmmac: use netif_set_real_num_{rx,tx}_queues

In the submission of the lastest multiple buffer patch set, this fix was lost.
I am sending this patch to put it right again. The fix was originally proposed
by Arnd Bergmann.

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