greybus: interface: fix interface-registration race

Fix race with user space when registering the interface.

The interface was registered before having been fully initialised,
something which could lead to user space accessing not-yet-initialised
attribute values (e.g. zero vendor and product ids or empty vendor and
product strings).

Note that this is also needed to be able to let attribute visibility
depend on manifest data (e.g. interface unlock).

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