Can we please change this name? I think Tomo is waiting for Andrew to
give his OK. All the other getter functions already follow the Rust
naming convention, so this one should as well. I think using
`is_link_up` would be ideal, since `link()` reads a bit weird in code:

     if dev.link() {
         // ...
     }

vs

     if dev.is_link_up() {
         // ...
     }

is_link_up() is fine for me.

	Andrew