greybus: interface: register control device at hotplug

Make sure to register also the control device along with any bundles
when registering an interface.

Note that we currently ignore failures to register the control device
just as we do for bundle devices.

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