greybus: interface: make interfaces children of host devices

Make interfaces child devices of host devices.

The new interface device name is "<bus_id>-<intf_id>", where bus_id is
the dynamically allocated bus id for the host device and intf_id is the
svc-allocated interface id.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>