mlx4_en: Fix error handling while activating RX rings

In case of failure of either srq creation or page allocation,
the cleanup code handled the failed ring as well, and tried
to destroy resources that where not allocated.

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