mlx4_en: get/set ringsize uses actual ring size

In case that the rings were not fully filled, the report in Ethtool
should be according to the actual size that was filled.

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