USB: serial: clean up usb-serial bus device removal

Make sure to unregister the tty-device before calling subdriver
port_remove.

This way remove will reverse probe, and specifically any port data
released in port_remove will be available throughout tty unregister.

Note that the order currently does not matter as the tty-layer can make
callbacks also after the device has been unregistered. This is
handled in usb-serial core using the disconnected flag, which is
already set when usb-serial bus device remove is called.

Cc: Peter Hurley <peter@hurleysoftware.com>
Reported-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>