net/mlx5e: Remove redundant check in get ethtool stats

ethtool core code makes sure data isn't NULL before calling
get_ethtool_stats, testing it again in the driver is redundant.

Signed-off-by: Gal Pressman <galp@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>