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>