staging: speakup: safely register and unregister ldisc

staging: speakup: safely register and unregister ldisc

This patch makes use of functions added in the previous patch. It
registers ldisc during init of main speakup module and unregisters it
during exit. It also removes the code to register ldisc every time a
synth module is loaded. This way we only register the ldisc once when
main speakup module is loaded. Since main speakup module is required by
all synth modules, it is only unloaded when all synths have been
unloaded. Therefore we unregister the ldisc once, when all speakup
related references to the ldisc have returned. In unlikely scenario of
something outside speakup using the ldisc, the ldisc refcount check in
tty_unregister_ldisc will ensure that it is not unregistered while in
use.

The function to register ldisc doesn't cause speakup init function to
fail. That is different from current behaviour where failure to register
ldisc results in failure to load the specific synth module. This is
because speakup module is also required by those synths which don't use
tty and ldisc. We don't want to prevent those modules from loading when
ldisc fails to register. The synth modules will correctly fail when
trying to set N_SPEAKUP to tty, if ldisc registrationi had failed.

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