link: fix type for link-config's "features" array of tristates

The "features" fields is parsed as a tristate value. The values
are thus not of type NetDevFeature enum but int. The NetDevFeature
enum is instead the index for the features array.

Adjust the type. In practice, this had no impact because NetDevFeature
enum commonly has size of int.

Also, don't use memset() 0xFF to initilize the int with -1. While
it works correctly in practice, it feels ugly.