staging: speakup_soft: Fix alternate speech with other synths

When switching from speakup_soft to another synth, speakup_soft would
keep calling synth_buffer_getc() from softsynthx_read.

Let's thus make synth.c export the knowledge of the current synth, so
that speakup_soft can determine whether it should be running.

speakup_soft also needs to set itself alive, otherwise the switch would
let it remain silent.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>