port_id(0x04): make invalid port printable
port_id(0x04): properly check phyiscal port range

OCaml does not provide unsigned types. The 0xffffff00l constant was
wrapping around to a negative value, causing the function to report that
all physical ports as invalid. Converting to Int64.t avoids the
wrapping.
Merge pull request #154 from frenetic-lang/port_id

Fix port_id checking