staging: speakup: Note that simple_strtoul can't simply be replaced by kstrtoul

We often receive patches which erroneously try to use kstrtoul in these
places.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>