USB: serial: ipaq: always register a single port

Use the calc_num_ports callback to ignore unused endpoints.

The driver binds to any interface with at least one bulk-in and one
bulk-out endpoint, but some devices can have three or more endpoints of
which only either the first or second pair of endpoints is needed.

This avoids allocating resources for unused endpoints, and specifically
a port is no longer registered for the unused first endpoint pair when
there are more than three endpoints.

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