Avoid reading num_tc directly from struct net_device, but use
the helper function netdev_get_num_tc.

Fixes: bc6a4744 ("net/mlx4_en: num cores tx rings for every UP")
Fixes: f5b6345b ("net/mlx4_en: User prio mapping gets corrupted when changing number of channels")
Signed-off-by: Alaa Hleihel <alaa@mellanox.com>
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>