USB: Digi AccelePort adapted to new tty buffering

this fixes the flushing trouble due to its own buffering for this driver.


Signed-off-by: Oliver Neukum <oneukum@suse.de>
Cc: Al Borchers <alborchers@steinerpoint.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>