USB: visor driver adapted to new tty buffering

the new tty buffering code allows usb drivers to stop private buffering.
In fact we must do so to allow flushing to work correctly. This does so
for the visor driver.

Signed-off-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>