net/mlx4_en: Limit the number of TX rings

Limit the number of TX rings per UP by the number of cores
in the system.

Signed-off-by: Inbar Karmy <inbark@mellanox.com>
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>