mlx4_en: fix allocation of device tx_cq

The memory to hold the network device tx_cq is not being allocated with
the correct size in mlx4_en_init_netdev(). It should use MAX_TX_RINGS
instead of MAX_RX_RINGS. This can cause problems if the number of tx
rings being used is greater than MAX_RX_RINGS.

Signed-off-by: Kleber Sacilotto de Souza <klebers@linux.vnet.ibm.com>
Acked-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>