userspace: port_state: rename a field, unused up to now

Rename "index" to "fiber_index", because it was never used, and it will
be the fiber index (enumerating fiber type 0..3). By reusing a field
of the same size, I don't need to change version number nor ppsi
(that uses the structure, but doesn't care about this value).q

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>