mlx4_en: set number of rx rings used by RSS using ethtool

Value must be a power of 2 due to HW limitation.
Driver supports only 'equal' mode in ethtool and can't be set by using weights.

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