staging: speakup: flush tty buffers and ensure hardware flow control

This patch fixes the issue where TTY-migrated synths would take a while
to shut up after hitting numpad enter key. When calling synth_flush,
even though XOFF character is sent as high priority, data buffered in
TTY layer is still sent to the synth. This patch flushes that buffered
data when synth_flush is called.

It also tries to ensure that hardware flow control is enabled, by
setting CRTSCTS using tty's termios.

Reported-by: John Covici <covici@ccs.covici.com>
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>