USB: fix cdc-acm regression in open

cdc-acm needs to set a flag during open to tell the
tty layer that the device is initialized

Signed-off-by: Oliver Neukum <oliver@neukum.org>
Cc: Marcel Holtmann <marcel@holtmann.org>
Cc: Paul Martin <pm@debian.org>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>