mlx4_en: Using real number of rings as RSS map size

There is no point in using more QPs then actual number of receive rings.
If the RSS function for two streams gives the same result modulo number
of rings, they will arrive to the same RX ring anyway.

Signed-off-by: Yevgeny Petrilin <yevgenyp@mellanox.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>