staging: speakup: check for null before calling TTY's flush_buffer

We should check the flush_buffer method of a tty for null before
invoking it. Some drivers such as usbserial don't implement
flush_buffer. This will be required for upcoming patches where we expand
spk_ttyio to support more than just ttyS*.
Signed-off-by: Okash Khawaja <okash.khawaja@gmail.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/speakup/spk_ttyio.c

@@ -227,7 +227,8 @@ static unsigned char spk_ttyio_in_nowait(void)
 
 static void spk_ttyio_flush_buffer(void)
 {
-	speakup_tty->ops->flush_buffer(speakup_tty);
+	if (speakup_tty->ops->flush_buffer)
+		speakup_tty->ops->flush_buffer(speakup_tty);
 }
 
 int spk_ttyio_synth_probe(struct spk_synth *synth)