mlx4_en: Setting actual RX ring size

When configuring HW resources, the RX ring size that is passed should be the actual
size that depends on number of buffers that we succeeded to allocate.
The mask for ring size should also be configured accordingly

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