speakup: Reference synth from tty and tty from synth

speakup: Reference synth from tty and tty from synth

We do not actually need speakup_tty and spk_ttyio_synth global
variables, the synth can store the pointer to the tty, and the tty
ldisc_data can store the pointer to the synth.

Along the way, we can clench the initialization of the synth and the
creation of the tty, so that tty is never NULL. Even if the device
disappears (e.g. USB unplug), the tty structure will still be there,
and we automatically stop speakup in the spk_ttyio_out error handler
but keep tty until the user cleans things up.

As a result, this simplifies locking a lot.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Link: https://lore.kernel.org/r/20210126222147.3848175-3-samuel.thibault@ens-lyon.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>