greybus: svc: clean up deferred hotplug handler

Rename the hotplug request message, and clarify that the message size
has already been verified by the primary handler.

Also add a debug message that includes the interface id.

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