greybus: interface: separate initialisation and registration

Separate interface initialisation from registration of the interface and
its bundles.

This is a step towards registering also interfaces that failed to
initialise (e.g. a dummy interface).

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Jeffrey Carlyle <jcarlyle@google.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>