net/mlx5e: Fix ethtool rxfh commands when CONFIG_MLX5_EN_RXNFC is disabled

ethtool user spaces needs to know ring count via ETHTOOL_GRXRINGS when
executing (ethtool -x) which is retrieved via ethtool get_rxnfc callback,
in mlx5 this callback is disabled when CONFIG_MLX5_EN_RXNFC=n.

This patch allows only ETHTOOL_GRXRINGS command on mlx5e_get_rxnfc() when
CONFIG_MLX5_EN_RXNFC is disabled, so ethtool -x will continue working.

Fixes: fe6d86b3 ("net/mlx5e: Add CONFIG_MLX5_EN_RXNFC for ethtool rx nfc")
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>