syscons: drop keyboard index from softc

syscons: drop keyboard index from softc

Analysis seems to reveal that sc->keyboard >= 0 implies sc->kbd != NULL and
there's no such scenario where sc->kbd is set (and theoretically used to
rebuild sc->keyboard) with the keyboard unavailable.

Drop the index softc. The index is only explicitly needed in few places, in
which case we can just as easily grab it from sc->kbd. There's no need for
keeping sc->kbd and sc->keyboard in sync when it can be readily accomplished
with just the former.