USB: cdc-acm, use tty_standard_install

This is a piece I missed the last time.

Do not copy the functionality all over the tree. Instead, use the
helper the tty layer provides us with.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Acked-by: Oliver Neukum <oneukum@suse.de>
Acked-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>