net/mlx5: remove erroneous fallthrough

This isn't a fall through because it was after a return statement.  The
fall through annotation leads to a Smatch warning:

    drivers/net/ethernet/mellanox/mlx5/core/en_ethtool.c:246
    mlx5e_ethtool_get_sset_count() warn: ignoring unreachable code.
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>

drivers/net/ethernet/mellanox/mlx5/core/en_ethtool.c

@@ -243,7 +243,6 @@ int mlx5e_ethtool_get_sset_count(struct mlx5e_priv *priv, int sset)
 		return MLX5E_NUM_PFLAGS;
 	case ETH_SS_TEST:
 		return mlx5e_self_test_num(priv);
-		fallthrough;
 	default:
 		return -EOPNOTSUPP;
 	}