USB: serial: fix generic chars_in_buffer

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: Pete Zaitcev <zaitcev@redhat.com>
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>