USB: serial: omninet: clean up port setup

These devices use the second bulk-out endpoint for writing. Instead of
using the resources of the second port structure setup by core, use the
new endpoint-remap functionality to simply ignore the first bulk-out
endpoint. This specifically avoids allocating resources for the unused
endpoint.

Note that the disconnect callback was always redundant as all URBs would
have been killed by USB core on disconnect.

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