speakup: Fix wait_for_xmitr for ttyio case

This was missed while introducing the tty-based serial access.

The only remaining use of wait_for_xmitr with tty-based access is in
spk_synth_is_alive_restart to check whether the synth can be restarted.
With tty-based this is up to the tty layer to cope with the buffering
etc. so we can just say yes.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Link: https://lore.kernel.org/r/20200804160637.x3iycau5izywbgzl@function
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>