speakup: add the missing synth parameter to all io functions

So that we can avoid the spk_ttyio_synth global variable in the next
commit.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Link: https://lore.kernel.org/r/20210126222147.3848175-2-samuel.thibault@ens-lyon.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>