USB: serial: fix interface data use

The interface data should not be used as a flag to signal disconnect.

Now that all serial drivers use the usb_serial disconnect flag and
mutex, we can set the interface data prior to registering the ports and
there's no need to clear it at disconnect.

This should hopefully also make it more clear that the interface data is
not a flag, which could prevent future misuse.

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