greybus: connection: fix potential null-deref on create

Make sure that the interface lookup helper can handle static,
bundle-less connections without oopsing when creating further
connections.

Note that the initial svc-connection has always been bundle-less, but
did not trigger an oops as a bundle was created for it before further
connections were created.

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