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*.

---
 drivers/staging/speakup/spk_ttyio.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

@@ -227,7 +227,8 @@ static unsigned char spk_ttyio_in_nowait
 
 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)