USB: serial: use tty_port_register_device()

USB: serial: use tty_port_register_device()

We already have the tty port when probing a usb-serial port so use
tty_port_register_device() directly instead of tty_port_install() later
to set up the port link.

This is a step towards enabling serdev for usb-serial (but we need to
determine how to handle hotplugging first).

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>