cdc-acm: fix crash if flushed with nothing buffered

Under some circumstances acm_tty_flush_chars() is called
with no buffer to flush. We simply need to do nothing.

Signed-off-by: Oliver Neukum <ONeukum@suse.com>
Reported-by: Torsten Hilbrich <torsten.hilbrich@secunet.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>