USB: serial: rename tty-port callbacks

Rename the tty-port callbacks using a common prefix to more clearly
separate them from the tty and usb driver callbacks.

Rename serial_down to serial_port_shutdown to match the callback name.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>