staging: speakup: add functions to register and unregister ldisc

This patch adds the above two functions and makes them available to
main.c where they will be called during init and exit functions of
main speakup module. Following patch will make use of them.

Signed-off-by: Okash Khawaja <okash.khawaja@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>