net/mlx4_en: Increase number of default RX rings

Remove limitation of netif_get_num_default_rss_queues()
from logic of RX rings default number.

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>