staging: speakup: change semaphore to completion

In this driver, both function the same way, but we want to eventually
kill off semaphores, so a completion is the better choice here.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>