mlx4_vnic: use netif_set_real_num_tx_queues to dynamically change tx queue size

mlx4_vnic: use netif_set_real_num_tx_queues to dynamically change tx queue size

When network admin vnics are created the network device is
already registered and it is not allowed to change
dev->real_num_tx_queues directly.

This fixes a bug where the unload of mlx4_vnic hangs with
the following error message: waiting for eth442 to become
free. Usage count = 16

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
(Ported from Mellanox OFED 2.4)

Signed-off-by: Mukesh Kacker <mukesh.kacker@oracle.com>