staging: speakup: fix a bug when translate octal numbers

There are actually overflow bug and typo. And bug was never happened due to the
typo.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>