mlx4_en: Verify number of RX rings doesn't exceed MAX_RX_RINGS

Required in cases were dev->caps.num_comp_vectors > MAX_RX_RINGS.
For current values this would happen on machines that have more
then 16 cores.

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