staging: speakup: migrate bns to tty

Migration of bns was missed out in the patch
https://patchwork.kernel.org/patch/9727725/. This patch does it by
updating relevant function pointers, just like in the patch linked
above.

Signed-off-by: Okash Khawaja <okash.khawaja@gmail.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>