net: honour netif_set_real_num_tx_queues() retval

In netif_copy_real_num_queues() the return value of
netif_set_real_num_tx_queues() should be checked.

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>