greybus: hd: move CPort allocation to host-device code

Move host-device CPort allocation to the host-device code.

Proper CPort allocation requires knowledge of the hardware and must be
handled by the host-device driver. This is an intermediate step that
moves the generic CPort-allocation code to the host-device code.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>