USB: serial: allow subdrivers to modify port-endpoint mapping

Allow subdrivers to modify the port-endpoint mapping by passing the
endpoint descriptors to calc_num_ports.

The callback can now also be used to verify that the required endpoints
exists and abort probing otherwise.

This will allow us to get rid of a few hacks in subdrivers that are
already modifying the port-endpoint mapping (or aborting probe due to
missing endpoints), but only after the port structures have been setup.

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