net/mlx4_en: Align behavior of set ring size flow via ethtool

In current implementation, any requested RX/TX ring size value
that is less than minimum is silently casted to nearest valid value.
Update this behavior to align with mlx5 behavior by printing warning
in dmesg and remaining the size unchanged.
Kernel is responsible for verifying against the maximum.

Signed-off-by: Eugenia Emantayev <eugenia@mellanox.com>
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>