USB: serial: mxuport: register two ports for unknown devices

Print a message and register two ports for interfaces for which we do
not know how many ports there are instead of binding, allocating
resources, but not register any ports.

This provides a hint that anyone adding a dynamic device id must also
provide a reference id (driver info) from which the port count can be
retrieved, for example:

	echo <vid> <pid> 0 0x110A 0x1410 > new_id

Signed-off-by: Johan Hovold <johan@kernel.org>