LinkType.LayerType() returns 0 for all types

In the following snippet, the LinkType().LayerType() method always returns a value of 0 for all layer types. It should return type=1 for example, when the link type is "Ethernet" or 113 when the linkType of the pcapgo.Reader is "Linux Cooked SLL"
```golang
pcapReader, err := pcapgo.NewReader(r)
if err != nil {
return "", err
}
layerType := pcapReader.LinkType().LayerType()
```