greybus: svc: register svc device at hello

Register the svc device upon reception of the HELLO request.

The SVC HELLO request contains the endo id and AP interface id, which
will be exposed from the svc device rather than the endo.

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