net/mlx4_en: Fix wrong limitation for number of TX rings

XDP_TX rings should not be limited by max_num_tx_rings_p_up.
To make sure total number of TX rings never exceed MAX_TX_RINGS,
add similar check in mlx4_en_alloc_tx_queue_per_tc(), where
a new value is assigned for num_up.

Fixes: 7e1dc5e9 ("net/mlx4_en: Limit the number of TX rings")
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>