Make sure chars_in_buffer accounts also for data in host stack queues.

This fixes the problem with tty_wait_until_sent returning too soon at
close which could cause the final write urb to be cancelled.
Reported-by: NPete Zaitcev <zaitcev@redhat.com>
Signed-off-by: NJohan Hovold <jhovold@gmail.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>