USB: serial: move pl2303 hack out of usb-serial core

Some pl2303 devices require the use of the interrupt endpoint of an
unrelated interface. This has so far been dealt with in usb-serial core,
but can now be moved to a driver calc_num_ports callback.

Note that we relax the endpoint requirements checked by core and instead
verify that we have an interrupt-in endpoint in calc_num_ports for all
devices so that the hack can first be applied.

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